hello sir, ulcer is a break or discontinuity in the epithelium . causes of ulcer are any injury due to sharp teeth or sharp food , gastro intestinal distress reflex, stress, nutritional dificiency  like vitamin dificiency. a ulcer may persist for 5 to 7 days . you can use dolo gel 4 times a day , and start with vitamin B complex. if it persist longer do consult a dentist