So I just watched another
video from Gravity Transformations. It discusses "nerd neck" which is generally caused by too much sitting. When he described the symptoms and the exercises to treat them, I did some of the suggested stretches, and boy let me tell you! When I stretched my neck I could really feel it right where I'm having pain.
Case solved.
It is definitely my rhomboids, which have been relaxed and unused for more years than I can count. Stretching my neck according to his recommendations helped—a lot—for a few minutes, and then the pain returned. My guess: After I stretched the pain went away, but then the muscles tightened right back up. Bent-over rows, low rows, and other backwork have been aniling those little suckers and they aren't used to it, so they're throwing a temper tantrum.
At least I now know how to stretch them out.
Also, since joining the Y I've had almost no time for stretching, and I need to fix that.
Just FYI: This Gravity Transformations dude has a lot of very helpful videos that I wished I had discovered on day one!