sounds like a simple mouth ulcer (or at least thats what we call it) just try to reduce the amount of acid in your mouth -- chew some Tums, Rolaids, etc. They do taste awful but help.

What does it look like? A whitish hole in your gum? If so its just a simple ulcer from mouth acid. If its a blister or something else, i have no idea how to help you. Hope this helped!