Thread 'Possibly infected prolapsed vent' https://www.backyardchickens.com/threads/possibly-infected-prolapsed-vent.1587069/
The pic in this thread is a prolapse.
Have you washed the bottom and removed the poop?? Please do. The vent are will probably be red and irritated just like bad diaper rash. If you have vetricyn- spray some on the vent area after you've cleaned the area well.
