An oral ulcer which is painful is most likely an aphthous ulcer. We usually get these ulcers during periods of stress like exams,etc. These are also seen in people with vitamin deficiencies. These ulcers though painful are harmless and disappear without treatment in a week. Please remember that ulcers due to cancer are always painless to begin with. Hence I advise my patients that a be happy if you have a painfull ulcer than a painless one.