If you have been biting on the inside of your mouth there might be some cuts or bumps. Or it might be a cole sore. You might get one from not brushing your teeth or sucking on your thumb.
Well it depends: I have a painless lump on the base of my neck but that's part of my spine! I'm guessing it is the same for you. But if you're really worried and think it is something different, then...