This deffinately doesn't sound like Pano.
I have seen what my cousin went through when her GSD had it.It started with his front leg and lasted a couple of weeks.He could not walk on it very well.Then it went to the other.Then it was worse when it went to the back.He had a fevor,couldn't eat,lost weight and didn't move around.With him it started at 5 1/2 months.It's also more common in males then females.
I agree with growler.It could be a muscle strain.
My suggestion,hold back on the hard exercise and try to control her zoomies.I know,it won't be easy..LOL..Go for extra walks.Then if the next morning she's fine when she gets up,then you will know.
I have raised 3 GSD's.My current is 12.I have never exercised them hard till they were around 14 months.This way their bones are much stronger.And this was suggested to me by my breeder when I got my first GSD.