For the time being, I would treat her for wry neck, which can include vitamin E 400 IU, human B complex 1/2 tablet crushed into food, and a little scrambled egg for selenium. Can you get a picture tomorrow of her from the front?
Well, can you look her over for possible wry neck or a spinal deformity? Wry neck can make the chest appear larger on one side. A spinal deformity can make the backbone look crooked or give the chicken a funny posture. Chickens from Rural King and most feed stores are not vaccinated for Mareks.
When you touch her swollen neck, does it feel solid or does it feel puffy like air under the skin? Was she vaccinated for anything? Her poop is very green, and looks like she is not taking much food. I would offer some scramble egg bits, wet chicken feed, and you can also try tuna or canned cat...