I know exactly what it is now. I went to my chiropractor and he quickly found all the tensed up muscles (of which there are several, specially on the left side...and wow, did they hurt when he massaged and stretched them!) It's tension in the muscles of the rotator cuff in the left shoulder. There is a tendon or a muscle involved which became irritated (either from the computer work or the punching into thin air bit or both). The irritated muscle becomes inflamed and swells up a bit, causing further irritation whenever you move your arm into certain positions, because the muscle (or tendon, can't remember exactly the terminology) passes through a narrow space between the bones and when it swells it can get pinched between those bones when moved in certain positions, and that hurts!

So, he worked on those areas some to loosen things up, and showed me an exercise to stretch and relax the muscles. He says good posture while working seated is essential to relaxing the muscles. He also recommends using an ice pack to initially reduce the inflammation and bring down the swelling. After that is accomplished, which may take a few days, use heat to relax the whole area.

This is good. It's a big relief to know what is going on, and this is a tense muscle problem, nothing more than that.

Joe, you made a smart move when you married that chiropractor. ;-) (in my opinion)