So while I can't rule the painlessness of my back out as a factor, there are lots of other reasons for feeling that I'm not actually an impostor into the adult world of competence today. Just saying.
Voltaren update: Day two, no pain. I can feel it niggling round the edges and *wanting* to hurt. When I stood up this morning my back went "I really should get onto that hurting thing, but I can't be arsed." I suspect that this morning would have been one of the ones where I'd have to lie on the floor to put my undies on, if not for the Voltaren.
I think that means it's working. I'll call the doc and make the blood test appointment today.