Impacted crop? I have a friend whose chicken looked like that with an impacted crop. It came back after they were able to clear it the first time. It was very low also and all of the feathers were gone too. I could be totally wrong just looks very similar to their chicken. Hope you get it figured it!!!
I feel silly. I was not looking at the picture right, and didn’t recognize the crop. Yes, I would agree with Nicrom that it appears to be an enlarged crop, and possibly impacted or pendulous. Check it again early in the morning to see if it is still full. Does it feel firm, doughy, or puffy. Impacted crop, sour crop, or pendulous crop are all problems that can occur in chickens. She may be pecking at her feathers there because of discomfort. Is she drinking a lot of water? Does she lay eggs? Here is some reading about crop problems: https://www.backyardchickens.com/ar...w-to-know-which-one-youre-dealing-with.73607/