Please see an oral medicine specialist in person.

A mouth ulcer is the loss or erosion of the delicate lining tissue of the mouth (mucous membrane).  The most common cause is mechanical injury, such as accidentally biting the inside of your cheek. In most cases, mouth ulcers are harmless and resolve by themselves in seven to 10 days without the need for treatment. If your mouth ulcers don’t clear up after 10 days, or if you get them frequently, see your dentist.

Please consult a dentist to diagnose properly as any non healing ulcer should be checked. If it comes and then heals before coming again then it could be related to a gastric issues