Answer by Jake Godfrey:

You are having an autoimmune reaction to certain proteins in milk. This is different from lactose intolerance. The problem begins with your general health and intestinal permeability. Here I introduce to you a theory much in debate, but still espoused by many good doctors, and so worth considering imo.
In what is called 'leaky gut syndrome', poor health (esp. poor tone of the abdominal wall) creates a state in which macro molecules may prematurely pass through the intestinal lining and into the bloodstream where they raise a general alarm leading to an inflammatory response and general adaptation syndrome.
Here's where it gets interesting. Milk proteins share certain properties with membrane-bound proteins on the insulin-secreting pancreatic cells called the Islets of Langerhans, which make it impossible for your immune system to tell the difference between the two. Antibodies rallied to mark milk proteins end up marking the Islets, and your immune system ends up destroying these cells.
That's right, milk may be the cause of juvenile onset diabetes. It is also theorized that milk may be responsible for all diabetes, and those who suffer juvenile onset have a particular sensitivity. If this is true, your reaction may be placing you at high risk for this disease. If I was in your position, I would remove milk from my diet, yummy though it is.
Leaky Gut Syndrome: What Is It?

Why does my body hurt when I drink milk?