Hi, it may be due to reverse smoking
, if u do, or may be due to a mouth burn due to hot food or liquids. The yellow appearance may be due to pus formation with the ulceration. Please apply Hexigel ointment as frequently as possible. The ulcer may heal in a week's time. Please consult an oral surgeon if it doesn't subside in a week or so. Some ulcers are precancerous. so do not neglect it. Regds!