Mouth infection? what could this be?

Sounds painful and because of the rash, sounds like something you are eating is irritating your mouth in some way (possibly an allergic reaction) as well. Can you think of anything out of the normal that you've eaten since this started - say a new brand of yogurt or different type of snack? Could possibly be an infection of some type from biting the inside of your cheeks as well, but you would need to go to the doctor and let the doctor advise you on that one. Very difficult for anyone to advise you on the internet, as it's definitely something one would have to see in order to tell exactly what's going on.