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It could be due to anything. Perhaps some acupuncture or some massage to the area may help? Yes I often have pains but I do stretching to help it the best I can.

Maybe due to the sleeping posture...

I agree with you that it might b e your sleeping position. It might also be the mattress that you are sleeping on, not much cushion perhaps? I also agree with you when you said that there might be something wrong. I did have that pain when my rotator cuff was not totally healed. It was actually inflamed 3 years ago. I had to go to the doctor and he recommended that I have xray. I did and thank God nothing was serious. I underwent a month's rehab for the muscles and tendons to get strong again. So with your pain right now, better go to the doctor so you will know what to do. Good Luck

Help is a very faithfully work