We have two Isa Brown hens that have been laying for around 6 weeks consistently giving one egg each per day. A few days ago one layed an egg with a thin shell and hasn't layed since. She walks around with her tail down and has become a little lethargic compared to what she was. She is still eating but not with any enthusiasm. She doesn't appear to be in any discomfort but isn't happy and active like before. We haven't changed anything and the second hen is still laying fine and is active and happy. Any ideas?