I suspect your gut bacteria are simply unused to the new environment (possibly including the milk). The same kind of thing happens to most people when they travel overseas. As a child I spent most summers in Mexico. Within the first few weeks I would have a bout of serious diarrhea and vomiting, literally every year. But native Mexicans who ate the same things didn’t have the same problem, being well adapted.
Anyway, what specific type of milk are we talking about? Full fat homogenized milk? Skim milk? Unhomogenized milk?
In the German case you might also be talking about UHT milk, which is common in many countries (not sure about Germany) but vanishingly rare in the USA? I think that stuff is awful, YMMV.
I suspect your gut bacteria are simply unused to the new environment (possibly including the milk). The same kind of thing happens to most people when they travel overseas. As a child I spent most summers in Mexico. Within the first few weeks I would have a bout of serious diarrhea and vomiting, literally every year. But native Mexicans who ate the same things didn’t have the same problem, being well adapted.
Anyway, what specific type of milk are we talking about? Full fat homogenized milk? Skim milk? Unhomogenized milk?
In the German case you might also be talking about UHT milk, which is common in many countries (not sure about Germany) but vanishingly rare in the USA? I think that stuff is awful, YMMV.