It is a lack of digestive enzyme that breaks down lactose, or milk sugar. I have written an article about IBS. Which basically is what you have. https://hubpages.com/health/IBS-Irritable-Bowel-Sy...
Qbc is a good digestive enzyme, although more for protein.
A lactase enzyme might be a better choice, Sold as :Lactaid Caplets, Lactase Enzyme Supplement, 120-Count Box. Available just about any where. See photo.
I actually had to quit drinking it for a while. I just couldn't tolerate it. After taking QBC for about 3 months the intolerence has gotten better.
Another option is to take probiotics, to straighten out your digestive tract, also explained in the article.