Originally Posted by xMSx

Do you smoke tobacco or dip?
If not then it is most likely a simple canker sore (assuming you don't have a fever) that can occur on your gums. The fact that it looks like a pimple means that you might have bit it in your sleep and that is causing most of the pain. It should run its course, but if it doesn't go away in a week, you might want to go see a doctor. Swishing hydrogen peroxide will be disgusting, but will clean out the sore and maybe prevent infection (it will turn any abrasions in your mouth white, however, for a little bit). Best of luck.