I have had one here with something similar. No visible damage to the leg, but was hopping. I separated her and put a baby aspirin in her water. No improvement but was still eating and drinking well. I checked her leg compared to the other leg and the only difference was she would hold the injured leg out straight and the toes were also straight ahead sometimes or curled other times.
After watching her for a couple of days it looked like her hock was not holding her weight. I think she might have injured it jumping down from the roost or caught it in something outside. A little vet wrap around her whole leg has given the leg more support and it is healing. Slowly.
It took me several days to figure out where it was injured.
Best wishes with your hen, this might be way off from what your girl has, but just thought I would post the information in case she is acting similarly.