It looks like she has been pecked by the other chickens. Are they overcrowded? What are you feeding? I would spray her bottom with BluKote to hide the red skin, or temporarily separate her. I would say the same for the neck if it looks similar, but a picture would help. Overcrowding , boredom, not getting enough protein can all lead to pecking and bullying. Are they still in a brooder?
Here's some more pictures I only have 8 chickens and we have a huge shed turned into a coop 6 nesting boxes two feeders and I feed them every other day as well as get them water everyday
The neck are looks enlarged. Is it maybe a full crop? Can younlocate the crop and also compare with the others’ crops? A leaking air sac in the neck would be a possibility, if it feels like air under the skin. Can you confirm that the red spot on her bottom is under her vent opening?