It could also be nerve damage. Go to the doctor if you can.
Hey, Doc. It hurts every time I move my arm like this.<br />
Don't move your arm like that.<br />
(Seriously, see a doctor!)
yeah, you should probably get off your computer and go straight to the doctor before you make your injury worse, or start trying to self-diagnose.
You might have sprained it or pulled a tendon. You should go to a doctor and see if you need muscle relaxants or x-rays.