Chicken unable to walk - any ideas?

I have a chicken (barred rock) that has been unable to walk for some time. We don't know what caused it in the first place, but have separated her from the others as they have been picking on her a bit. She doesn't seem to be in any pain, but one of her legs just straight up isn't able to support her weight. She's also pailer than usual. We did notice a small grayish bump on her ankle in-between her scales, and it wouldn't come out. It was not on the leg that was ok. If you need an image, I can provide one. She also won't eat her normal chicken food, but that just might be because the other chickens have been bullying her there. If anyone knows caused this or how to treat it, I would really like to know- Thanks!

Hello This does sound serious.  So far, this could be a huge range of things, from a wound to arthritis to a parasite, abscess or a fracture, perhaps with mites as a secondary problem.   It sounds as though she needs to see a vet.

One of the possibilities however is a disease called Marek's, which causes almost all the symptoms you mention.  The eye can change also; the pupil get smaller. Marek's is difficult to treat and infectious, so if this seems likely it is extremely important to take care of your biosecurity and to speak to your vet ASAP