I've been riding a max distance of 200km rides for only about 4 yrs now, and while my stomach has never been the most robust on these rides, it's not been as bad as on a 200km (DIYxGPS) back in May this year.
It was a 210km/+1000m, and I'm fairly sure this was down to food choice, as I made the mistake of having a heavy, thick cheesy coated moussaka for lunch, and it just sat in my stomach. I became more and more uncomfortable, and drinking warm water for teh bottle made me feel worse. I had to stop more often, and then ~2 miles from home, well, it all came up! Although, unusually for me, this 200km followed a w/e ride and full weeks commute to work of ~200km. so perhaps this put me more 'on the limit' than usual.
On a 110km/+2000m ride the following month, I had a more restrained cheese roll/crisp/choc lunch. But the warm conditions and extent of the climbing, made be feel a tad grim early afternnon, so I had to sit in the shade for ~30mins, and felt relatively Ok to continue after that.
Last w/e's 120km/+900m with a substantial pre-ride breakfast, brownie at coffee time, and pork pie ~12pm, was absolutely fine.
So for me I think it's trying to cope with food not digesting, when I've pushed myself more than usual for some reason. My next 200km will be a tester for having a few days breaks from the bike before, eating little & more often, and avoiding anything that will take up residence in my stomach.