Shoulder injury (again)

Hi Dr 8a! I have a problem with my right shoulder, like a knot under my shoulder blade. It has been there consistently for about a year. I feel it could be related to climbing, while others have told me it could be from using a mouse at work... it has been very hard for me to isolate the exact spot in a stretch, but I discovered it by accident last night: If I extend my right arm out to the side along a desk, and then pull with my right hand on the edge of the desk, while at the same time using my left hand to pull my head left and down, it strongly isolates the affected muscle, so much so that it makes me dizzy! Is there anything you can tell me about which muscle I'm isolating and what the problems/solutions could be? Dustin.