I had a simliar prob the other day with leftover lentil soup I made. Had been in the fridge for about 5 days, a no no. Oh well. I ate it and had a huge amount of bloating, gas and diarrhea that followed til it was cleared out of my system. I know I have read tha bloating can be die off but it is also a sign , like you have said, that we are feeding the little &^*(&%))_'s and it is fermenting. After all that is what yeast do is ferment and cause gas , which causes the bloating sometimes is a sure sign of fermentation.
Many years ago my father made his own wine. He used to put the bottles on their sides to ferment , only he put them in our dining room in a case that kept it dark. One night, in the middle of the night we heard what we thought were gun shots, it was the cork flying out of the bottles. We could have been killed by rapid firing wine bottles, lol. IN any case , during fermentation the gases build up and whammo, the corks pop. Of course this was in my Dad's amatuer days, lol. And as a matter of fact, that wine was not fit for anything but pouring into a car as fuel or using for paint remover. Man was that stuff nasty and harsh. We still joke about its possibilites. Raisen wine...yuck.
Oh, the point was that the soup must have contained mold and then yeast.