One of our chickens has had a very slight and occasional limp which has suddenly got a lot worse. She still walks fairly normally until she has to do anything like moving sideways, running etc., when she clearly cannot put weight on her left leg. She doesn't hold it up although sometimes she looks like she's trying to give it a rest. She is also still scratching for food but sometimes this causes her to lose balance. She is eating fine, drinking fine and is laying ok too. We've checked for bumblefoot and all clear. Any ideas on what to do next?