You don't say where in the arm it hurts. Is it the shoulder? The elbow? The wrist? The biceps? Where it hurts would help in telling what's going on. I have a similar problem, but I have a lot going on so nobody's ever really been able to pin things down for me. I've been told my arm pain, which comes every time I go back to lifting weights, is from disc problems in my neck, from my shoulders, and uh, I dunno. That's where the docs always come out, I dunno.