It is difficult to say exactly why you are biting your cheek without a thorough clinical examination. The 2 most likely reasons are 1) a bruxism habit (grinding/clenching), which is notorious for causing linea alba (white line) inside the cheeks or 2) a supererupted upper molar (usually a wilson tooth). There are other possible etiologies as well. In any case, the final diagnosis should only be made chairside. I hope this helps.
Dr. Robert D. Lipschutz=20