One of my hens is 22 months old and has enjoyed good health. Lately, though, she has been sitting in a corner of the cage, all puffed up. She typically runs around the cage with her sister and comes to the door when I bring worms, but she hasn't moved much. I can't find anything else odd. She is eating a little and drinking water. She laid an egg yesterday. I just can't figure out why she's doing this. Any ideas?