You are probably clenching your jaw without knowing it. Is it worse in the morning? I do this but it has gotten tons better than it was. Keep track of it, if your teeth start to hurt, talk to your dentist. He can fit you with a splint that fits over your teeth. It doesn't keep you from clenching, but it spreaRAB the pressure out. Do a search on teeth grinding or bruxism. You will find a lot of information on it and things to do to relieve some of it.
