My jaw keeps locking
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
http://www.essortment.com/all/whatistmj_…
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