Showing signs of aggression towards some people

Wondering if anyone can provide some insight into a recent change of behaviour,
Kaya is 8 1/2 months now, she is very friendly with other dogs and usually humans.
There's been 2 incidents today and yesterday which have had me worried.
Yesterday we were out at the seaside on a picnic bench and there was a man a few yards away from us, standing on the spot staring at the floor singing loudly to himself. Kaya didn't like him at all, she very rarely barks but she was howling at this man, pulling towards him and growling, we moved away because I couldn't settle her, I put it down to his unusual behaviour and decided I'd have to work harder on kaya's 'stop' command, hoping it was just a one off.
But then today 2 children (10-12) walked past us and she done the same thing.
Its not like her at all and I'm wondering if its a change in her behaviour possibly because of her age? Everybody else she's gone past today and yesterday she's been her usual friendly self towards.
The other thing is we've been on holiday for a week and she is absolutely shattered! maybe she's feeling cranky.

Dogs go through a second fear period at about 6 - 14 months and so it is vital that careful socialisation and habituation is conducted.

Could she be coming into season?

Ah that is useful to know,I remember her going through a brief fear period before, she didn't show aggression that time though.
Yes, she could be coming into season.
I'll have a read up on fear aggression, maybe something about the children spooked her and I didn't notice.

I was just going to echo what has been said above really as it does sound as though she is going through some kind of change - be it a physiological change such as her first season or a psychological change. Worth keeping an eye on but nothing to get too worried about just yet. Keep socialising her as widely as possible, keep up with her training and with any luck in another month or twos time, all will be fine again.

Are you going to get her spayed? If so, worth a chat with your vet about it now if she is coming into season so you know when to book her in for.

Thanks for advice, yes she is getting spayed, there is a possibility she's coming into heat although I haven't noticed any other signs.