Cheek biting is commonly associated with insufficient horizontal overlap of upper and lower teeth.Seeing a dentist is advised.
This is very common, and it's difficult to heal because of where it is and you do keep biting it. You can have the sharp tooth filed down a bit to solve your problem. To help it heal, rinse your mouth 2-3 times a day with hydrogen peroxide, and spit it out. You can also coat it with milk of magnesia which protects it from further irritation and will help make eating less painful. You have to be very mindful of this and try not to chew on that side, and chew slowly and carefully while you have it and until you can have the tooth filed down.