Have your dentist, oral surgeon or ENT evaluate this. In the meantime, avoid hard food, smoking or rough brushing. It may be a virus or something worse. Have it evaluated soon.

Photo is not very clear but it probably can be treated with an antiviral Rx by a Dentist or physician. I’m guessing Herpes Simplex Type I.
If so it can be transmitted, so avoid touching it until diagnosed treated, and healed.

Looks like a recurrent apthus ulcer, a type of virus that lasts for 7-10 days. I've had some success in breaking the cycle by rinsing twice a day with original listerine (yellow) for several months.

Dr. Moser

Not enough information. You need a full medical history. Possibly trauma related.