Can be because of sharp teeth, or stress.

Mouth ulcers can occur due to many reasons. The most common ones are constant lip/cheek biting, sharp tooth/ filling edges, trauma/injury of any other kind, poor oral hygiene. Cause has to be evaluated in order to get permanent relief. Meanwhile you can start with a non alcoholic mouthwash once daily before bed time after brushing, brushing twice daily with tongue cleaning. Visit a dentist to check for sharp edges causing trauma if any.