Sudden diaper rash is evident in babies who are suffering from diarrhea. The baby's diaper is frequently changed, but their skin gets moist repeatedly by being washed. The more it's washed, the more it's wiped by a baby wipe, and a new diaper takes place, making the skin around the bum chafe or irritate. If you're tightening the diaper too much, that also contributes to developing a rash. Try changing the diaper brand.