Why is my shoulder sore?

My shoulder is very sore; actually, the muscle that lies behind the collarbone, trapezius I believe. It has been sore for several days and is most noticeable when I am still. It doesn't seem to be restricting my movement any, it is just sore. It feels like it won't relax. I'll try to relax it, but it still feels tense. I slept in a different bed for about a week, and it started hurting after that, but I have been back in my own bed for three nights now. I can't really afford to go to the doctor. Any suggestions?