Maybe you've become lactose intolerant? I've got a few friends who cannot digest whey protein because of that. They had to switch to egg white protein. They experienced the same symptoms as well. Just a thought..
Is it possible that you had a stomach flu?
As far as 90% of the world being lactose intolerant: That's an exaggeration. A lot of non white people have trouble digesting milk products, along with a small amount of white people. People (all races) as they age tend to lose the enzymes needed to digest milk sugar, so the problem can develope later in life.