Moulting, Limping Hen

One of my hens is losing lots of feathers and is hobbling. These things aside, she is eating, drinking and behaving as normal.
We have started her on an antibiotic (we have some broad spectrum stuff that the farmer gave us).
The other hens are fine.
Any ideas?

I have just had a hen hurt herself coming down from her perch, we have lowered it, and we are going to put a ramp up, she has been limping scince saturday, she is almost better today, i hope this will stop the problem, T C