This is an incredibly common condition when skin is inflamed by diaper contents.

Time with diaper off and barrier creams are the most important considerations.

Barrier creams prevent the skin from being affected by the liquid or solid contents of a dirty diaper. They become a literal BARRIER or block against the irritating substances. 

Change diaper frequently!  Do not let baby sit in a wet or soiled diaper for long.  Every second counts!

My favorite barrier creams contain products like ointments, beeswax or zinc oxide.  Use them if the skin seems pink, red, sore or inflamed or before bed.  Motherhood taught me that in this case, as with everything, be GENEROUS in your approach.