In most cases, a canker sore will only cause symptoms at the site of the sore. When a canker sore is beginning, you may feel a tingling in your mouth -- a sign that a sore is going to emerge. This will usually progress to a bump in the mouth that is bright pink or red. The spot develops into an open sore, or ulcer, that is white at the center and red or pink around the edges. Within a few days, it should go away.
In some cases, however, aphthous stomatitis (the medical term for canker sore) will cause more general symptoms throughout your body. These include a general feeling of discomfort (called malaise), swollen lymph nodes, and fever. These unpleasant symptoms should get better within a week or 10 days. If you notice symptoms (or the sore) for longer than two weeks, call your doctor.