Canker sores are a common form of mouth ulcer that are painful and typically seen on the tongue, inside of the cheek, lips, etc.
In many cases the cause is unknown and they go away in a few days without treatment. They can be triggered by stress
, foods, hormonal changes, and vitamin deficiencies. You may want to follow up with your physician to make sure they are not something more severe.
Answer assisted by Randy Burgett, Medical Student