Everyone has explained the gas and bloating. The second problem is diarrhea.

That's because you can't absorb lactose through the intestine. The molecule is too big and humans don't have specific transporters for it. This means you get a high amount of lactose in your bowels.

Physics tell us things always strive to go from a high to a low concentration, essentially diluting it. If the lactose can't move from your bowels into the bloodstream, it will instead drag lots of water from the blood into the intestines.