Does your keyboard have a num pad? If so, then you're likely extending your arm to to far, bending your wrist at an odd angle, and relying on your shoulder to bear the weight of your arm when you use your mouse.
Thank you for your suggestion. This is certainly a contributing issue when it is the case that I'm using a keyboard with a numpad and a mouse together, but not the only one.
I use many different keyboards and pointing devices (not just mice) and the general trend I described holds true.
