I've Been Waking Up With Clenched Fists Over A Period

I've been waking up with clenched fists over a period of months. Recently I've noticed clenching during the day as well. I have an under active thyroid and take medication for that but am not on any other. The pain in my forearms from the clenching is now interfering with my work and functioning. Any ideas what could be causing this or ways to alleviate the pain?