Usually goes away
What you describe ranges from being an ulcer, to an allergic reaction
, to an area of trauma.
If it grows every day for 5 days or more, or if it doesn't disappear in 10 days, you should visit your doctor. Usually these just disappear in a short period of time. Try dissolving a teaspoon of table salt in warm (soup temp) tap water and swish it inside your mouth for 5 minutes twice daily x 5 days.