According to gastroenterologist Dr. Kathlynn Caguiat, "Alcohol can increase gut motility and it doesn't get broken down before it reaches the colon, where bacteria feast on these, resulting in bloating and diarrhea." Those bacteria love the alcohol you feed them, and they repay you with extra gas and crap.
Dirty draft lines can completely change a beer's taste and even make the drinker sick. What's in My Beer Lines? There are a few different elements you might find affecting your draft lines. Yeast, bacteria, mold, and beer stone are your most likely culprits.