My one arm is really sore! For no apperant reason!?

Okay, I'm 26 years old, and in relatively good health. I woke up this morning, and my right arm is so sore, like the kind of sorness that you get if you get a shot, or if you get punched in the arm. But neither one of those things happened to me! It's also on the right side of my neck too. At first I thought maybe I slept on it funny last night, but now I'm starting to have my doubts. The pain is not easing up, and I can't really bend or move my arm without being in pain! I mean it's not so painfull that I want to cry, it just is so darn sore, it's very unpleasant. I took asprin, but it didn't work. And now I have noticed that when I walk, the arm hangs down a little lower then my other. So what do you think it might be? Should I be concerned or not? Please help!