Sounds like a posture issue to me. Be sure you aren't "reaching" up, try to keep your shoulder itself in place and not stretch the arm up as if you trying to get something in the back of a cabinet
Also be sure when you are moving your arms backwards you are turning your body completely to the 'hug' position, not turning completely causes your shoulder to turn/twist in a bad way. I had a big problem with shoulder pain until I watched myself on a webcam recording. Recording yourself just to look for bad posture is very helpful because so many times I kept falling back to bad habits.