I agree w/Ginger.....What your describing is classic of an abscessed tooth....OUCH!!!
Please, see a Vet to have this tooth extracted and antibiotics started....It's very important to NOT let this get out hand....If the infection continues, it will eventually enter the bloodsteam....There is great potiential of Heart damage, then Liver & Kidney problems from these organs trying to filter the toxins (Infection).
Then the PAIN!!!! It will lead your dog to not want to eat...This then leads to all other problems, too....
See your Vet for a Doggie Dental....It will be worth your time and money......Karla
Has your dog had a recent injury, or cut there? If so it might have got infected. But a likely cause of this would be a tooth abscess. He/she needs to see a vet.