Mouth ulcers. Mouth ulcers can be caused and/or triggered by different things, including infections, autoimmune diseases, allergic reactions, nutritional deficiencies, and stress. The most common kinds are canker sores (aphthous stomatitis), the cause of which is not known, and cold sores caused by type 1 herpes simplex virus. You should be evaluated by your doctor and treated accordingly.
A virus. Mouth ulcers are caused by a virus. The two most common ulcers are canker sores and herpes ulcers. Canker sores occur on the soft llining tissue of the mouth like cheeks, inside lips, and under the tongue. Herpes lesions usually occur on lips. These ulcers are brought on by trauma, or spicy foods, or stress. They can't be cured. Symptoms are treated with ointments or with lasers. See your dentist.
Herpes(canker sores) The most common reason for mouth ulceration (other than chemotherapy or other major health events), is irritation, and of course the more common outbreak of herpetic lesions (viral infection). As i was taught in school, without use of anti-viral medications, they will resolve in 7 days with treatment, or a week without treatment!