Hello, You may be biting your cheek and have caused some trauma to the tissue. Do you think you grind your teeth? This may occur at night. You may notice inflamed tissue in a line next to where your upper and lower teeth meet. Congestion, mouth breathing or snoring may cause a dry mouth and make tissue more sensitive. Other conditions can include a broken tooth or growth. Schedule a routine dental exam and discuss this concern with your dentist. A night guard can help control a grinding habit. Rinsing with warm salt water, maintaining a soft diet and avoiding irritants will assist tissue healing. Keep well hydrated, follow a proper diet with balanced vitamins and continue to practice good daily dental care. Thank you for your inquiry.