I suspect a bacterial infection. But you still need to rule out a crop disorder. https://www.backyardchickens.com/articles/how-to-treat-sour-crop-and-impacted-crop-and-how-to-know-which-one-youre-dealing-with.73607/
Do you have any antibiotics on hand? Leftover people prescriptions will work...
Have you checked her crop in the morning before she eats to see if it's emptying over night?
What color is her poop? How about a pic?
Where do you live?
Is she laying? Any soft eggs recently?
Is there a drippy discharge from the vent?
Is she making wheezing sounds? Have you opened her...