I have a discoloration of my inner cheeks.  One side more than the other.  It is not white or red.  It is dark.  I do bite my inner jaws.  This is a habit that I have had since childhood and I am 38 now.  No problems.  It is not painful or anything.  I would not have noticed had I not just opened the inside to look at the back of my throat. What could this be?
Not to worry. You can have diffed. Color skin inside. Still I would suggest going to your dentist and get it checked. And adjust the causal so you don't bite your cheek. In long run that tissue can thicken.