It's from gritting my teeth! I was going to a dentist that my husband really like, and he didn't have a clue as to what was causing the problem. I decided that I really didn't like him anyway, and went back to a dentist that I like better, who uses more high-tech, modern approaches to dentistry, like air abrasion instead of drills. (Although those darned gumline cavities require drilling

I know that others around here grit there teeth as well, so I want you to be aware of the problem. My dentist is going to treat this very aggressively because one those start it's very hard to get them under control. He's using a filling material that fights tooth decay.
I've been using a Somnoguard anti-snoring mouth guard at night and that has put an end to any nighttime teeth grinding. And when I have problems during the day, I use a sports mouthguard or stick a cork from a wine bottle between my teeth.
Oh and dry mouth from meds doesn't help with this problem either, or the fact that so many meds cause us to clench our teeth.