I hope she's ok - I'm trying to find an article on ball chasing/fetching which I saw online a couple of weeks ago but am failing miserably so I'll precis it from memory!
It's very likely she injured herself fetching her ball - the motion of running flat out and then braking, twisting and turning to get the ball is not a natural movement for a dog and they do place massive stresses on their joints and can cause many injuries. Retrieving in water is better and less stress on them, or restrict to only a few ball throws per walk.
It's much better for them to 'find' things and slow down and use their nose rather than chasing a ball.
I hope she's ok and has just twisted something that is easily fixed