It seems possible that you are experiencing a canker sore, which is a result of too much acidic foods in the stomach. This usually happens when I eat too many tomatoes or drink too much coffee. They are painful, and you can buy some topcila over-the-counter creams to apply locally at a pharmacy.

The sore could also possibly be an outbreak of HPV (herpes) --- so you may want a doctor's diagnosis and opinion.

Good luck!