Since the x-rays showed nothing, it could be neurological in nature.

My Muskrat (Tortie in avatar) was a rescue that had a previously broken leg which did not heal properly, thus she has a permanent outward "bow" in her hind leg, which causes her no pain and does not impede her ambulation at all . . . she can run, jump and play with the best of them! When I trapped her, she had a bone infection in that once-broken leg, and for awhile there was the possibility that amputation might be the only alternative since the antibiotics were not working, but my vet was finally able to cure the infection and save the leg. Even 3 legged cats seem to not notice their handicap and adjust quite quickly and very well, so I was not concerned.

Hope everything turns out just fine for you and your kitty!