Sudden pain in left arm?

I couple of hours ago I started having pain in my left arm and it hasn't stopped. It hurts a lot when I move it, so much that I could almost cry. It hurts from my finger to my neck. So all the way up my arm, over my shoulder and a little up my neck. I didn't hit it or anything, it just happened all of sudden. I tried laying a pillow under it while laying down, and then it stopped after some time, but it started hurting again as soon as I sat up again. Should I be worried?