Based on the picture that you have posted, this seems to be a case of cheek biting..... if it is chronic and recurs frequently, you need to get your dentition evaluated by a dentist so that any correction if required can be planned..... you can apply anaesthetic gel like dologel CT for symptomatic relief..... Regards.....

Hello..picture is suggestive of cheek bite.kindly visit dentist for further evaluation.For symptomatic relief,apply Dologel over the red area.

This can b due to cheek bite while chewing, and may be your upper  teeth are more towards  cheek making  them slight out of alignment... Your upper or lower  back teeth may have  slight sharp points which  aren't leading to frequent cheek bite.. Go to ur near by dentist . Meanwhile u can apply an ointment like  dologel-ct or Anabel or rexidine m Forte gel... Try avoiding spicy food..  Do slight warm saline rinses...