I had a pullet who was limping, and I couldn't see anything wrong either. The problem resolved by itself. My guess was she "landed wrong" when she jumped off the roost, but the problem healed in a day or two.
She seems to be putting a little more weight on it today but still limping. After a little investigation, my son admitted to trying to catch her to hold her. Though sweet intentions, I think she injured her knee in the process of escaping. Beyond sad. I read I can give her baby aspirin for pain. She is eating and drinking. What more can I do?