Megan's hind leg??

We have had megan now for about 5 weeks, we got her from rescue, she is a 12-15 month old staffie. For the past week every so often she suddenly yelps in pain then 1 of her back legs goes all stiff (it looks like cramp). It only lasts for about a minute, then it more or less seems to go back to normal. I have also noticed that she doesn't sit straight either she tends to sit on her side, she has always done this though. Just wondered if anyone had any thoughts on what this could be please??

Thanks.

I would have the vet take a look it could be a luxating patella.

http://dogs.about.com/cs/disableddogs/p/luxating.htm

Could be a ruptured cruciate ligament. Does she sit with the leg out straight? Frump ruptured his, and after the yelp of pain, he then used to carry the leg for a while. Due to his age, the vet would not operate, but gave him Metacam and advised rest. After a few weeks, he was able to weight bear on it, but still always sat with his leg out to one side.
Toffee had luxating patella, and was operated on for it last year. She didnt yelp in pain, but I believe some dogs do. We only noticed it as she started to hold her leg up in the air for a few steps before putting it down again and "flicking" it as if to straighten it. The op was huge, but very successful.
Wish you all the best, and hope Megan is getting on ok otherwise, but definately a vet visit I'm afraid.