Hi Sir,
It looks like a cheek bite, which is because of wisdom teeth. This is a usual occurrence as, wisdom teeth rarely erupt correctly, any buccal tilt, there will be cheek biting. Don't worry at all. But do the required treatment.
Next Steps
visit a dentist and take required action
Health Tips
use mucopain gel and apply over the area and leave there for 10mins before rinsing. (just be careful and take the medicine after showing to a dentist)