My 3 week old chick can walk dont know what to do?

I have a cornish rock and she cant walk at all. She was just fine when we got her about when she was 2 days old. But now she scoots around the bottom of the box with legs spread out. I hate seeing her in so much pain when she trys to move. Shes still eating and drinking just fine. Our other 7 chicks are doing just fine so that's why we dint know what is causing it. Could use some help on this. My hubby and I are first time chicken owners.