Did I do muscle damage to my shoulder/arm? I can't lift my arm above my head without pain...?

I know I did this during exercise, but I am concerned about the severity. The muscle in my arm, between my upper arm and shoulder, is in pain. It hurts to lift it above my head and other stretching hurts, too. I thought it may be because I was exercising a new muscle, but after a week, after the rest of my body is fine, this one area hurts a lot. Any advice?