Border Terrier being aggresive on walks

Hi, I was wondering if you guys could offer some help/advice for an issue I'm currently experiencing with my BT. Her name is Jessie and she was 2 in November.
In the past 2 weeks on walks she has changed her behaviour. All incidents have been offlead (she has been offlead since a puppy and has good recall) Basically she attacked an older smaller dog around the neck. No dog was injured but she has since done it another 2 times on different walks at different times. I have gone back to basics with her and begun practicing recall in the park with clicker and tasty rewards (which I do on and off but not consistantly), something I know she will come back for. Nothing has changed at home. I haven't changed her routine at all or her food. I'm not really sure what to do next. I do have another dog a Cavalier King Charles who is also 2 (was 2 in May this year) but they get along really well always looking after each other, lots of licks. No untoward behaviour between them. I do have a 4 year old son too and we practice sit, click and reward with both dogs. We have wooden dog games where the wooden pieces slide back and forth for the treat which we use from time to time.
I take her to agility once a week and there is a shnauzer that is quite hyper and makes lots of noise when the other dogs are going round and she has in the past tried to attack this dog, maybe it stems from there? Ideally I want to be able to change the behaviour. Otherwise all I can think of is a muzzle or lead walking for the time being whilst working on this new behaviour she has, but asides from what I'm already doing I just wanted to know if I should be trying anything else. I forgot to mention I have introduced a ball when on walks to try and get her attention back on me which is working but if she decides that she doesn't like another dog, which thinking about the past 3 incidents it seems to be old dogs, she attacks them around the neck after a stand off (the other dog today wasn't doing anything only wagging tail and sniffing side by side).
Like I say this is new behaviour and she has never done this before in fact she loves playing with all dogs old and young before this new behaviour begun. I will add she will not come away when she is being aggresive I have to pull her away. My CKC did the 1st and 2nd time join in by chasing the dog away after my BT had attacked it (no bites but definitely not playing or being nice). Today I noted that my CKC was interested in only the ball and did not join in with my BT. I have re-read my post and I don't know if I have used the right words to try and explain what my BT is doing so please feel free to ask me any questions and I will try to elaborate or make clearer. Just to re-iterate there is no biting when my BT does this but I am worried that there might be if I don't muzzle her - otherwise she won't get any exercise either which I'm thinking will make it worse. My BT gets one walk a day for about 40 maybe 60 mins in woods or park (mostly woods). Thankyou.

Maybe a chat with Borderdawn on here would help?
I once had a very aggressive bitch who would insist that other dogs deferred to her (trying to avoid the D word here) and there is a condition where a bitch can get too much testosterone if she lies between 2 males in the womb. So she ends up very very pushy.
It isn`t common, but it does sound like you`re doing all the right things.
Or she could just be a bully. (sorry).
If she were mine I`d walk her with a trailing line and recall her around other dogs - and also muzzle her until the behaviour is addressed.

She sounds just like my terrier girl Lily. She got on with all dogs large and small until she was about 18 months. She would do just what you describe, no biting, but it looks and sounds horrible and is clearly unacceptable behaviour.

I can control Lily in certain situations, onlead she tends to be absolutely fine, but as I know she has this streak in her, I avoid letting her make offlead contact with potential targets. It means I have to be super vigilant on walks but it also means that she gets her offlead time, plus she gets to interact with other dogs. I am very choosy over who this is though and will only let her offlead if I am sure that she can't get herself into trouble.

Her targets tend to be small, white, fluffy and female, although not always. Having said that, I fostered a Maltese girl and Lily got on fine with her. The initial meeting was done with Lily muzzled and onlead. As Daisy is a confident girl, it all went well and the two still meet up and get on fine.

If you have an unpredictable dog you have to be on the ball and prepared to monitor their doggy meetings at all times.

Firstly, I would from today keep her on a long line or Flexi.As the recall fails when she is being aggressive, you simply cannot take the chance of your dog injuring another dog. It's only a matter of time before another dog owner makes a formal complaint. Using a muzzle for now is a good idea too just in case another dog comes racing over and your girl reacts aggressively.

I think I would try and get help from an experienced trainer. I would also suggest getting your vet to check your girl over - as from what you describe the aggression is a recent thing, it may be that something physical is triggering it...

If the vet gives her the all clear, then I think a trainer or behaviourist is the way forward. There may be subtle signals going on between your dogs, or between your dogs and other dogs, at the park that you as the owner are missing, which someone obective might pick up on.

Do let us know how you get on?

Hi, thank you for your advice I really appreciate it.
I thought a muzzle might be appropriate until the behaviour is addressed. Do you think a full plastic muzzle or one of those soft muzzles? I have looked and there are few muzzles around and I've never had to muzzle one of my dogs before so it's new to me
I will get a flexi or long line and continue with my click and treat rewards.
Can you ever trust them again offlead though (once the behaviour has been worked through successfully)? She played with a Staffie and was absolutely fine yesterday but the older dog she attacked.
Although a behaviourist is a good idea my funds are limited at the moment so I think I will muzzle and use the long line for now, although I do fully appreciate that an objective point of view would help.
Jessie has recently recovered from kennel cough. The first incident happened just a few days before she started coughing. She has since been given the all clear by the vet.
I'm wondering if I walk them seperately if it will help (my dogs that is). I agree it is hard work being one step ahead of them and it makes it increasingly difficult to do this when you have your young son with you but it is possible. As from Jan my son is going to school full time so I will have more time available to spend with Jessie working through the unwanted behaviour.
I too agree that I would not want this behaviour shown towards any of my dogs so yes it is right to put her on a flexi or long line and muzzle. She does have lots of energy and will happily walk for hours so she will become more frustrated due to limited exercise and become more boisterous around the house (ie playing with my CKC). I'm still wondering if I should continue with agility? It's very hyper tense for the jumping. Collies are the noisiest but so too is the Shnauzer and last week a collie snapped in her face, she didn't do anything. My instincts tell me there is an upside to agility which it the focus on me but the downside is that the atmosphere and general mood of the dogs there (including Jessie) is heightened excitement and they are hyper too. Should I stop it temporarily see if it helps? Mind you saying that we did stop for several weeks due to illness in the family and then my son caught chicken pox. So thinking again maybe the atmosphere doesn't affect Jessie in the way that I'm thinking...
Thanks again for all of your advice and all other advice is very welcome too - thankyou.

Just wanted to add that on lead she does nothing it's only offlead

You want the basket type muzzle, the fabric ones that hold the mouth completely shut are not suitable for exercise as they don't allow the dog to pant.

Some great advice so far, hope it helps.

Good on you You are a truly responsible dog owner; there are many that are not, but you are doing all the right things so I really wish you all the best and DO keep us posted?