What I am about to tell is going to be sad. It will probably make you cry a little bit. You will not want to believe it, and even with the awarenss of it's truth, you will probably rebel sometimes; but I know you more than you can imagine, and I know that you will always regret your rebellious attitude. Always. Brace yourself, because here it comes: You are lactose-intolerant. Milkshakes will always make your tummy hurt.
Stop eating them. I'm serious.