Why do I always seem to get a stomachache right after drinking milk?

I don’t think I’ve had this problem before as a kid or young adult.

I don’t know about chocolate milk, I don’t drink it that often.

I can eat cheese, ice cream, yogurt, sour cream and cottage cheese just fine without issues. It’s only milk that affects me. I only drink milk in cereal and with cookies but I still get stomachaches that usually go away in the same day.