Thanks for the update, Igy78, your input is really helpful. It's good you got specific problems diagnosed and know how to approach them.
I've had stomach issues for a long time, but I never associated them with POIS, since 1) I had them long before my POIS started, and 2) many people have stomach issues, can't tolerate acid foods, etc. What is interesting, though, is that 2 weeks before my POIS started, I had acute stomach problems, diarrhea and vomiting, as a result from eating eggs that had gone bad. Maybe those spoiled eggs added something to contribute to my microbiome? Who knows what was in them. I used to drink cola but have quit it completely about 20 years ago (with POIS starting about 4 years ago).
I have problems with acid food, like tomatoes - my belly inflates, gets filled with some sort of gas and I feel some type of expansion from the inside taking place. This disappears if I lay on my back for some time, and I've had it long before POIS, that's why I've always thought of it as an unrelated and more minor problem.
An interesting observation: there is a certain NSAID, dexofen (active ingredient is dexketoprofen), which, despite being anti-inflammatory, on its own seems to activate my POIS! Well, it causes stomach disturbance (any time I take it) + activation of POIS (most of the time). What do you think? On the one hand, NSAID's should have positive effect on our POIS. But here, even days after orgasm, this triggers POIS if I take it. Maybe it irritates the stomach and that in turn brings about POIS symptoms? What irritates the stomach need not be the active ingredient, but one of the minor ingredients. That would be consistent with some kind of "gut-induced" theory for POIS
Can anyone propose a possible connection between ejaculation and activation of gut inflammation? I don't know enough internal medicine to imagine how the two would be related.