Why couldn't it just be a mouth ulcer?
If you're concerned, get it checked out by a doctor, or dentist.
Other things to try - ice. It can also numb the area and reduce pain.
If it's a mouth ulcer, you can try camomile tea (as a mouth rinse). I've also dabbed on hydrogen peroxide (I can't use those numbing gel things). Honey might also be worth a try - if possible get GOOD honey from a beekeeper, not pasteurised stuff from the supermarket. Honeycomb is good - chewing the wax afterwards, a bit like chewing gum, can also promote saliva and this also helps with antibacterial action.
That's for a mouth ulcer, anyway.
Marg