That would be a periapical abscess. The tooth it is associated with is dead, you should have a root canal or extraction a.s.a.p. Waiting too long will allow the recurrent infection to break down the bone supporting the tooth and the long term prognosis of a root canal will be diminished. Having a long standing recurrent infection can also cause numerous other systemic problems and risk blood sepsis if the infection is released into the blood stream. Go to your Dentist, get an X-ray.