Reasons of ulcers are- immune disorder, blood related disorder, infections (viral, bacterial, fungal), traumatic from your sharp teeth, Due to some medications. Vit deficiency, constipation, stress etc.
Next Steps
you can consult dentist or gen. physician with your all kind of medical history to rule out the exact cause..They may suggest some test if required.
Health Tips
As it is self healing in 7-15 days, you need to reduce the discomfort in present for that u can apply mucopain 3-4 times in a day 10 min. before food.