Turns out, I'm anemic.
Well, that's not what the doctor said. The doctor said my 'hemoglobin was down', and that I seemed to be 'holding on tight' to iron. I have no idea what that means. My Hb is at 131 (I take it that that's grams per liter), where 'normal' for men is 140-180. So a) I'm now on an iron supplement, b) I have to go off the iron supplement and follow a special diet for several days to do the Occult Blood test, and c) after that, it's off to the GI specialist.
Could be a diet thing. I did check through all my meds and none of them specifically mention iron absorbtion problems. Assuming I'm not bleeding internally or have any of the more dire possibilities, which are really very remote in the circumstances, the obvious solution is to eat more red meat. Yay. I love red meat. I always assumed too much.
I assume that has consequences for the cholesterol and the B12 (or one of those Bs, I forget which, which is only available through red meat or supplements). But whatever. I'm not worried. Except for the forthcoming adventures in medical testing, which I don't expect to be a problem, but won't be any fun.