Canker sore. Canker sores hurt really bad and they look like little white spots with a red outline usually.
Canker sore. A canker sore often occurs inside the mouth, and soft part of the mouth, such as inside the cheek, soft part of the roof, or even tongue. It is round and flat, or almost slightly caved in--"ulcerated" or ulcer-like--and feels like burning or quite painful. There may be one or more, and typically smaller than the top of pencil eraser head. They appear rather suddenly and disappears in a week or so.