I saw the physio at 8:30 this morning. Looking at what I had been experiencing, she agreed with my analysis that it's probably a nerve doing something wrong in my shoulder, and even narrowed down which nerve it's likely to be (addressing two nerves that would cause my symptoms). She gave me some mobility exercises, some strengthening exercises and some stretches to do for my shoulder and my neck, and said that if the problem's still there in 2 weeks, come back for a second appointment.
I forgot my gym bag, somehow, which had my belt and knee sleeves in it. My knees didn't like training without sleeves very much. Fortunately, I had no competition for the gym's communal belt (as is usually the case -- those of us who use belts usually use our own belts, with few exceptions), so I was still able to do the target weights for this session.
Decline Sit Ups
In the middle of the session, and again towards the end, I did several of the rehab exercises given to me. My left shoulder's feeling pretty good right now, but still not as solid as my right shoulder.