The exact cause of most mouth ulcers is unknown. Stress or tissue injury is thought to be the cause of simple mouth ulcers. Certain foods, including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes and strawberries), can trigger a mouth ulcer or make the problem worse.
Home remedies
step1-mix ghee (clarified butter) and turmeric powder.
step2- apply it to affected area.
if it goes in a day fine otherwise try for few more days.