I've been vegan for 8 months. recently i tried products with dairy in them and each time i got bad stomach pain and runny stool. why does this happen?

Lactose intolerance: Defects in lactose absorption can be acquired (due to inflammatory conditions of the intestine*) or congenital. Adult lactase deficiency (hypolactasia) is autosomal recessive (ar) ; present in >75% of non-caucasians ; 20% of caucasians. Congenital lactase deficiency (ar) is acute watery diarrhea in infants. *see your doctor for evaluation if your lactose intolerance is indeed of sudden onset.

Food sensitivity: The protein in cow's milk is 20% whey protein and 80% casein protein, and you may have sensitivity to the casein or whey or be lactose intolerant. Good to have this checked out by your physician. An integrative medicine doctor can run a simple test to check your sensitivities to these products.