It happens and it's maybe a bacteria in food you have eaten, rather than the food itself. In other words, its food poisoning. And it's common whether you have a J pouch or not. I remember when I ate a prepared antipasto salad I bought at a supermarket salad bar one time and it made me so sick and dehydrated that I actually passed out and found myself on the kitchen floor. It took me around 10 days to 2 weeks to recover. There was nothing individually in that salad I couldn't handle. What I couldn't handle was some bacteria in that preparation.
Apart from hydrating, when you have a relentless diarrhea like that you stop it by taking Pepto Bismol. Pepto Bismol crushes any diarrhea because it crushes the bacteria that is causing it. It's bismuth and no bacteria survives it. Attack the cause of the symptoms before you start treating the symptoms. Always have PB in your house. I do. After you take the PB and shut down the diarrhea then you can rehydrate. But don't let the diarrhea continue and just keep drinking. It's a losing strategy and you may end up in the ER using such a strategy.