What causes diaper rash

My mom has OCD. Every time I’m at her house, and my son needs his diaper changed, she freaks out thinking I never change him (I check it every 30-60 minutes.) and it makes me feel like a horrible mom if he ever gets a rash even though I know some foods can cause rashes like if he eats this baby food with pineapple or pomegranate I notice he gets a little irritated. But she goes off saying I don’t change him and it’s cause by not changing him quick enough. (He doesn’t go long with a dirty diaper on. Like 5min max)