Why does it smell like something crawled up my & died everytime I drink Hershey's?

It doesn't happen when I drink milk or eat cheese,...so,....I really don't think I'm lactose intolerant. Only after I drink Hershey's chocolate milk. It's SO delicious!! It's just so good! But about 2 hours after I drink it,.... the most putrid stench begins seeping outta my bowels! It's bad,...flowers wilt & goldfish rise lifelessly to the surface of the bowl.