I have a buff Orpington chicken who is 4 months old. She has been lethargic since yesterday morning but is worse today. Today she won't eat or drink and has not left the coop. The temperatures have been in the low to mid 20s the past two nights. Our other chickens are fine and active. I put a heat lamp in the coop today to warm her up but it hasn't helped her. What do I do?