I did some searching regarding your problem. It seems to be a common,problem for people. Most of what I could find were saying that it is a TMJ issue or perhaps you are clenching/grinding your teeth at night when you are asleep.
Here is a web site I found that tells you what TMJ is
You may also wish to consult your dentist as to what is going on either way they can help treat you. If you do not have dental insurance or a dental plan here is a web site that may help you on that
I hopes this helps