Why is it that I feel sick?

Whenever it is hot outside,if I drink milk I get sick(stomach pains,vomiting). It doesn't matter if I am in an air conditioned room,then go outside after drinking it I'll get sick. I always have been able to get overheated easily. Typically I'm fine if I drink milk or have the items listed below in the winter,fall,or spring as long as it's cool out. Milk products don't get me sick though,ice cream,milkshakes,and just plain milk does though. So why is it that I'm getting sick?