Most mouth sores are cold sores or fever blisters, canker
sores, or other irritation caused by biting of the cheek, tongue, or
lip; braces or burning the mouth from hot food or drinks.
Cold sores are caused by herpes simplex virus and are very contagious. Usually, you have tenderness, tingling, or burning before the actual sore appears. Herpes sores begin as blisters and then crust over.