You most likely have a wisdom tooth which is impacted (stuck in bone due to lack of space), you will have to get it extracted. It will be a small surgery and you will have to take rest for a day or two after surgery. Medicines can be prescribed only after check up, in the meantime you can start with warm saline gargles.
Next Steps
Consult a dentist nearby
Health Tips
Get treatment started as soon as possible as the swelling may increase more