Sick Chick!! Please Help!!

Have a black sex link hen chick guessing about 4 weeks old. She has just started getting grown up feathers. Found her laying limp in corner of box this morning. Picked her up thinking she was dead. She wasn't but she was very lethargic. Put her in box by herself to see what is wrong. She has been eating, drinking and acting fine till this morning. Now she won't move, will stand if you stand her up but hangs her head down on floor. Keeping her eyes closed. Can hear a slight wheezing/rattle occasionally when she breaths. Won't eat or drink. Poo seems almost normal-went once in box all day. Might be slightly runny. No change in diet or water. No change in flock. Have 4 other chicks and a little turkey she has been with for about two weeks now. All others seem fine. No symptoms. Oh also I looked for mites or parasites and didn't see anything. I did see the skin on her stomach and under her wings seems red and kinda bare of feathers. She seems to kind of shiver when you move her around or try to get her to stand up. I've got her separated from others and nestled in a towel with heating pad on low under towel. PLEASE let me know what you think it could be. I'm new to raising chicks and am worried she is too weak to pull through whatever it is. Also worried the others could get sick too. Thanks in advance for your help!!