Well, as I got off the organ this morning, to go up for communion, I caught my foot in a carrier bag that had been left on the floor beside the organ ("out of the way" apparently - of everyone except the organist). I stumbled a bit, but thought no more of it. By early afternoon, however, my back was a bit sore, so I rested with a hot water bottle.
Let's just say that, after a further 70 minutes of near-constant playing this evening, the muscles down the right side of my back have gone into spasm. Pretty sure it's just muscular, but I could do without the pain. 🥴