I had a chick with a similar problem. Are you able to gently bend the leg? Or is the leg stiff? Is there any swelling? Are her toes curled under? Also, what type of surface is she on? If she has been on a slick surface with little grip, perhaps she fell and hurt her leg that way.
The problem could be a couple things.
It could be
1. she fell or hurt it herself
2. She has a neurological disorder
3. A vitamin deficiency
Also, is she shaking her head? A shaking head could be a sign of some sort of disorder.