Yep. I've been told its from TMJ, slight overbite as well, but I also used to grind my teeth as a child, and later began clenching in my sleep.
I also get wicked migraines. I've even found myself clenching throughout the day, and it does seem different from the muscle and jaw constrictions that accompany certain medications and stimulants. Those tend to be more long term clenching in the back of my mouth (molars and muscle clenching) whereas stress clenching usually happens for me in my front teeth and is much more noticeable and easy to recognize (and subsequently admonish myself).
I've found (from years of stress on top of add medications) that forcing yourself to keep your jaw slightly open, or placing the tip of your tongue between your teeth can help prevent mindless clenching and grinding.
Although once you go to sleep, there's no controlling that, lol.. I'd suggest getting a mouthguard.