New to keeping chickens

Hello everyone,

We have only had our Golden Comets for 2.5 weeks. One of our hens starting dragging her left wing and is limping, not moving around much at all. She is laying down a lot and has a lot of trouble when she does try to stand up. We examined the wing and it doesn't seem to be broken, she's eating and drinking, poop looks normal, and she's laying eggs, and her eyes look clear and normal. We've separated her from the rest of the flock, and I've given her a small amount of apple cider vinegar in her water. Any ideas on what may be wrong with her? Please help!