No leche in la noche

So I got sick for the first time after having my first smoothie in a gringo bar but more interesting was my host grandfathers explanation of why it made me sick:

Supposedly milk is not the problem.milk is fine if you drink it warm in the mornings but during the winter, which it is here, at night, you can't have cold milk? Still not sure why but apparently this is a well know fact. Kind of cool,