Mike, I also get gut problems for a few days after orgasm: Gas, feeling like I have to go number 2 when I don't, bloating, cramps sometimes.
If I avoid orgasm, my gut is manageable with the right diet. ("MCAS-friendly" diet works for me; Low-histamine, and avoiding histamine release triggers.) But after orgasm it'll act up regardless of diet.
I don't know what it is, but I'd guess in my case it's an inflammation of the gut, as I get a whole bunch of inflammation symptoms elsewhere as well.