VB-
I suffer from exactly what you described. It crept up on me over a 5 day kite trip. There was no hard crash or accident to blame, the only other factor was sleeping on a rather thin pad in my truck bed. And perhaps age- I'm 43. It hurt like hell. I self massaged then finally broke down, saw a physiotherapist. She said it was definately rotator cuff, gave me some exercises. It got more bearable but lingered, still flares up now 2 years later. Flareups seem related to kite days. Also take glucosamine Chondroitin supplements for it. Not sure if that helps, but anything to avoid that pain and/or surgery.