Lactose Intolerance
I have known people with this problem and have watched them suffer when they try to just be like one of the gang and eat some foods with milk and/or cheeses in them. Some of them told me they were "allergic to milk". So this is what motivated me to look into this more and do a bit more reading...
It seems as if there are millions of men and women that are not able to drink a glass of milk, other milk products, or even just a slice a pizza. This is not just a problem in the United States, but many other people in the world suffer from this problem! It's called Lactose Intolerance.
Symptoms vary but include nausea, bloating, stomach pains and cramping, diarrhea and just a general gassiness. So you're feeling pretty badly! 
This intolerance comes from a shortage of the enzyme Lactase. This is usually produced by the cells in the small intestines. Lactase is supposed to break down the milk sugar into a simpler form and then it should be absorbed into the blood stream. If there isn't enough Lactase to help digest the lactose that is eaten, it can end up like the symptoms that I described above...not very pleasant!
One source that I read actually said that a "Lactose Intolerance isn't an all-or-nothing condition. Some people who are genetically Lactose intolerant don't have symptoms, because their large intestines have adapted to undigested sugar. Others have found through trial and error that not all dairy products cause them discomfort. Yogurt is expecially tolerable because the bacteria cultures used in making it produce lactase. Research has also shown that many people diagnosed with lactose intolerance can still enjoy foods like ice cream and cheese, as long as they eat them in small amounts or with other foods."
Some of my sources for this article came from;
The Ladies Home Journal, or lhj.com
Diagnosishealth.com
gastro.org
Teri
"Because he is he and I am I."......E. V. Lucas