I used to get regular pain in my left shoulder blade similar to yours in your right shoulder blade. I wasn't masturbating then, though. I saw a doctor about it and he had no clue. Eventually it went away. However, as I recently discovered it was probably the first sign of fibromyalgia, which is a really nasty chronic disease. The pain comes and goes and is typically related to stress. So if masturbation is creating stress, it is the stress and not the masturbation that is causing the pain. I certainly hope, for your sake, that it does not lead to fibromyalgia. Fortunately, for me, I came down with it when I was 55. Some folks get it much earlier in life.