If your stomach hurts after drinking milk then you're likely lactose intolerant.
Most milk contains lactose which some people are intolerant too or have sort of like an allergy to the milk that contains lactose.
So if your stomach hurts after drinking milk then try some lactose free milk and see if that does not hurt your stomach and if it does not then you're lactose intolerant.
Lactose is basically a type of natural sugar found in milk in most mammals including humans and some people are just intolerant to that.
Lactose intolerance is not as serious as a regular dairy or milk allergy but the lactose intolerance can cause stomach pain, bloating etc.
When you're lactose intolerant your body has a harder time digesting the lactose or sugar found in the milk and dairy products which makes your stomach hurt.
Symptoms of Lactose Intolerance are stomach pain, farting, bloating, gas buildup in your stomach and abdominal cramping.