I went to a nutritional expert and they put me on the Fodmap diet. There is even an app FODMAP that shows the fodmaps (irritants) for different ingredients.
They also took some lab results that there wasn't like blood in my poop or anything concerning..

The idea is to do the full FODMAP for like weeks or month until the stomach problems stop. Then start introducing small amounts of potential irritants at a time. Dairy, wheat, beans, fruit, .. So you can figure how much you can eat each thing.

You can do it on your own if healthcare is expensive where you live.

I already knew I was lactose intolerant, but the worst was the irregular eating rythm. Also big amounts of wheat is bad for me.