Have you ever heard of chicken Bumblefoot? I cannot recall what that is exactly but it has to do with sores on the bottom of their feet. I would definitely keep her away from the big hens. I have a banty rooster and he had a difficult time living with the big guys. He's full grown now, and does ok with the hens... they are Rhode Island Reds. Is it possible for you to keep her in your home? Close by to watch and observe? Put her in a box, just keep feeding and watering and see if she improves.