You could simply have something small stuck between your tooth & gums and it became inflammed. It's happened to me many times. Once I retrieved the object, the pain begun to subside as well as the swelling. but, yeah...also consider it could be a secondary condition of a possible serious primary (angina, etc as the above poster said). See your doc first...rule out this with some blood work, etc and if you come up clean...go see your dentist if it continues to bother you.