When my sons were babies (34 years ago) one had a terrible diaper rash. Almost blister-like. I phoned the doctor who was in her 70's THEN. She told me that she could prescribe an ointment that would cost around $20 (back then that was quite a bit of $$) or I could follow the tried and true "old time remedy". I said I'd give that a try first. Worked so incredibly well that I never did buy the prescription.
Boil up a bit of water and put in maybe 5 or so regular tea bags. Take them out after about 15 seconds and let them cool completely on a plate. Once cool, pack the diaper with the tea bags and leave on for an hour or so. Then, remove the diaper and let the baby be "butt naked" for as long as possible.
The tanic acid in the tea is a natural healer. The air dry works wonders.
Please give this a try. You won't be sorry, and it costs practically nothing. I'm a grandma now and have given this advice to many a new mom and it always works.