You are not supposed to drink milk as an adult. Milk is for babies and children. No other species drinks milk after babyhood.
The main sugar in milk is lactose. Most people cannot digest lactose and land up getting diarrhea.
In normal human beings the enzyme that helps digest lactose is called lactase. Lactase stops being produced by the body between the ages of two and five. Once your body stops producing lactase and you drink milk, the undigested sugars land up in your colon. Here the sugars begin to ferment, producing gas, cramping, nausea, bloating, flatulence and even diarrhea.
If you are worried about getting enough of a calcium intake consume more leafy greens, beans, almonds, citrus fruit, strawberries, papaya, broccoli, sweet potato, turnips and carrots.