I`ve been suffering with exactly the same thing for several months. When I twist the arm or reach back behind me, it hurts like hell. The pain comes on during and after the movement and travels across the shoulder up into the neck and down the arm. It`s a really horrible pain and difficult to describe. I think it has come from years of sleeping on that side as the shoulder is sort of twisted underneath. I think it might be to do with the ligaments. Ibuprofen didn`t really make any difference. I am trying to sleep on the other side now and what has really helped is that over the last few days, I have hit it with regular doses of Naproxen. Maybe you could try that. (Unfortunately, I going on hols today to a country where I can`t take Naproxen into so it will be interesting to see if the pain comes back)