This is commonly known as a bite fibroma
, and develops after a person bites the inside of their cheek or even their lip.
If it does not go away you can have it removed. Your dentist can refer you to an oral surgeon who can remove the affected area, most likely under local anesthesia.
Also, if you do this frequently, then you may ask your dentist to smooth out any rough edges that may be present on your teeth.
I hope this helps. Good luck.