Gas, not enough enzymes, fatty foods...
Here's another thought that was brought to my attention by a nutritionist once.
Remember in high school when you did the experiment of mixing vinegar and baking soda? Boom.
Eating yeasts and carbs while eating proteins (meat and dairy) at the same time has the same affect in the gut as combining vinegar and b soda. Because they are essentially the same thing. If you have enough natural enzymes to break it down, no prob. In my case, I dont so I try very hard not to combine my proteins and my carbs. It's called the food combining diet. It can help with gas, reflux, and other GI issues.