Hello. The pain may be because of the eruption of the last tooth, wisdom tooth. Kindly share a photo for better diagnosis.
An OPG (full mouth x-ray) is required
You may have to get your 3rd molar tooth extracted.
Next Steps
Kindly visit a dentist for definite diagnosis and treatment
Health Tips
Lukewarm salt water rinse will provide temporary relief. Brush the area properly. Rinse your mouth after every meal.