>>725406362
In a very basic way, milk is harder to digest than cereal because it has more complex sugars like lactose, while most cereals, being grain based, are something we're pretty experienced at eating and adapted to. When you have one or the others, the mechanics are simpler, the compounds are already reduced to the "food cake" you masticated with cereal or the plain liquid you ingested, and the process is not complicated, its the normal process
When you mix both, you're having the digestive system act with a ball of cereals completely drenched in lactose and partially chewed food that's soggy as hell. Your digestive system is working overtime to work out the complexities of lactose digestions while also handling the normal fare of the "food cake", so it became a lot harder for it to multi task both including also processing the liquid versus just dry foods.
Think of it like having an army if you will. If you have one threat, its easier, if you have two at the same time and working together to fight you, its harder. This analogy is likely missing the fact you're likely intolerant to milk to some extent due to a lack of bacteria and such, so alone would be enough for you to handle it while with other stuff it might require a lot more work, so you'd have a shortage of specialized soldiers but basically think like that.