Hi.
Diaper rash is most commonly caused by irritation from stool and urine. If a baby diaper is filled with their stool or urine for a prolonged amount of time, it will irritate and cause a rash.
The Nurse should include that this is typical with newborns and babies. They will most likely cry when they need a diaper change because of the irritation in the 'lower' region.
Hope this helps! :)