Mine had the same thing and she ended up having ring worm. You should take it to the vet so he can tell you for sure.
My Pit Bull Has Red Bumps Under His Mouth...they Come And Go...sometimes They Bleed.......?
My pitt had the bumps under his chin too and I took him to the vet and they did a skin scrape and determined he was illergic to his plastic bowls I feed him in. They gave him antibiotics and gave me metal bowls and he has not had the probem since t has been 1 1/2 year with metal bowls now.