Hi. How old are you and when is the last time you could do this? Is this a movement you need to do or is the ONLY time it hurts is if you move in this position? I'd, obviously, try not to do the movement if you don't have to and see if it gets better. If it doesn't, we can get impingement in the shoulder, tendonitis or bursitis. This is usually if you feel pain at the tip of your shoulder and then down your arm. Since it happens when you move your arm the way you describe,it sounds to be related to the rotator cuff. See if it gets better but if it continues, get checked with your doctor. Rotator cuff injuries are not fun!
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. But again, could be you tweaked the elbow given the movement that is hurting. Could be something called biceps tendonitis. It's possible it's from the shoulder, but if it was it would hurt in the shoulder area. If it's nerve impingement rather than a muscle tweak or joint tweak, though, it could be coming from anywhere above that area, as shoulder, elbow, biceps, and neck problems can all cause inflammation that impinges on nerves that travel down the arm. If it bothers you, might be time to see an orthopedic surgeon and get some diagnostic testing done. If it doesn't bother you and as said above, you don't need to do that particular move, sometimes it's best to avoid docs on these things. You can also try icing, resting, and elevating for a couple of weeks and see if it helps.