Check the expiration date!
The other day I made myself a protein shake - same as I've done hundreds of times before. Within a half hour I was sick as could be - throwing up and also the other end. I thought I was dumping, but then I checked the expiration on my milk (which I don't use very often) and found it was quite outdated (over a week). I think I'm going to switch to soymilk - it lasts longer! I made the same shake today with water and everything was good. How stupid can I be lol :)