If you have given her a good inspection and you can't see anything visibly wrong with the leg, then it might just be that she is protecting it for some reason. I have taken in rescued turtles that wouldn't come out of their shell because kids had been teasing and tormenting them. Give her time and I think it will be ok. As long as she's eating and otherwise acting normal, I wouldn't worry about it. The other turtle might be pestering her too. Any number of reasons.