This happens sometimes when people have strokes or in a coma - the muscles shorten and go into spasm. But I'm sure that's not what you're referring to. Have you always done this or just recently. If it's recent it could be med related.

It might also be tension - where your muscles get rigid. My FIL used to have this facial expression where he sort of looked like he's smiling(showing his teeth), but not really smiling. Several of his children, including my husband does it. My youngest daughter does it and it usually happens when she's intensely concentrating. I'm going to ask her pediatrician what the exact term, for this. Try to become aware of the habit and correct yourself every time you do it. Good luck in finding a correct diagnosis.