It could be due to iron
deficiency anemia, pernicious anemia and other
B-
vitamin deficiencies, aphthous ulcers, oral lichen planus, yeast infection; exposure to irritants such as hot foods, or spices, tobacco, alcohol; allergy to toothpaste, mouthwash, breath fresheners, dyes in candy, dentures, food items, coloring agents, or blood pressure medications like ACE inhibitors. It could also be a mechanical irritation by a jagged tooth edge or a denture.
Treatment will depend on cause.