Why do I always have diarrhea 30 minutes after eating Taco Bell?

It doesn't make sense. I'm pretty sure 30 minutes isn't enough time for the Taco Bell to get all the way through my digestive system and be converted to diarrhea. That means whatever is coming out has to have been in my system beforehand. And if that's the case, why is it coming out as diarrhea? If I ate a healthy non-disgusting breakfast before eating the Taco Bell, then it seems like I should just have normal healthy poop in my intestines. So where does the diarrhea come from?