So I dunno

The shoulder is some better right now. Getting up and moving around (including doing ~30 minutes piano practice) seem to have helped it.

It feels like - from having looked at online anatomical diagrams - it's the deltoid that hurts. (So, I'm guessing it's either a pulled muscle, or also possibly a cramp. I tend to be prone to getting muscle cramps easily (I know, I probably need more potassium or something. I drink orange juice and I used to eat bananas before I developed a mild allergy to them).

I'm going to watch it, but if it continues to feel better, I think I can assume it was maybe a minor overuse injury when the polycart tipped over and made my arm twist into (what is for it) an unnatural position.

I probably also need to try to force myself to start doing yoga again; I have problems with not being very flexible and tending to hurt myself when I twist in funny ways.