Could i be lactose intolerant?

It could have probably been the cereal. Usually when someone is lactose intolerant they cannot eat anything that has milk for example ice cream that is a big no no if they eat that they will start feeling different and you didn't you said nothing happened. Another things that lactose intolerant people cannot eat is cheese, yogurt mainly anything with milk. Maybe you should trying eating a different cereal and if you get diahrea again it might be that the milk is not doing you any good.