You may be grinding your teeth in your sleep, which can eventually damage your temporomandibular joint. Check with your dentist to see if that is the problem. If so, he can make you a night guard (also called a night splint) to help prevent you from grinding your teeth.
You also may want to sleep on you back at night because sleeping on your stomach pushes your lower out of alignment and can cause futher inflamation to the temporomadibular joint! If you are short on cash you can even make your own nightguard by going to any sports store and getting a mouth guard that you would mold to your own teeth, probably a little more bulky but will do the job!!!
PS but go see your dentist first for a diagnosis!!!