It seems to be the bland stuff that my stomach rebels against. I had a bout last night, the second night in a row, of burping and general stomach discomfort, feeling bloated and gassy, and tried to think what it didn't like.
What was different? Bread. I had made bread in the bread machine, and made garlic bread on both Saturday and Sunday. And that started me thinking. Maybe, just maybe, it's still-active yeast that survived cooking? For that matter, maybe that's what's in instant mashed potatos, which wouldn't be in freshly boiled/baked potatos. Could this be fermenting in my stomach? Seems possible. Sourdough (not as active a yeast) doesn't do this, so maybe it's time to try sourdough with the bread machine. Tastier, too.