Lactose intolerance is pretty easy to diagnose. Once saw a Korean kid bite into a hamburger which had a dried milk byproduct in its make up ~ took a few seconds after it hit his stomach and he projectile vomited it from one side of the campfire to the other.That's not lactose intolerance. Lactose is digested in the small intestine. Milk actually makes food stay in the stomach longer (fat and protein content). So if the Korean kid vomited within seconds, it never reached his intestines and had absolutely nothing to do with whether he could digest it.
Actually some people ~ and the Koreans have more than their fair share of them ~ have a problem just as the first lactose containing substance begins to creep passt the pyloric valve ~ and it can happen quite fast.
Most of my closest relatives, and certainly the inlaws, have this problem. It can be a nightmare ~ seriously.
The rule is HUMAN BEINGS CAN BE AND ARE DIFFERENT!