It seems to be swollen under the skin. It did have poop stuck to it but after cleaning it. It still has a not under the skin. It seems like it hurts him for me to mess with it.
Clearer, up close pictures would be helpful. Can't tell what's going on. Is there injury, was it pecked at around the vent? How old is the chick and was it normal before? Did you hatch it, or purchase it?