8month old has her 1st diaper rash. ?

I have tried polysporin desitin butt paste baby powder. I've let her air her butt out. What else can I do. Grandmother said use corn starch. Its my grandmother and I know she would never do anything that would hurt my babygirl. Is corn starch safe?? This is her first diaper rash she has ever gotten. Yesterday she was pooping a lot. She had an upset tummy. I feel so bad I've tried all I know to do. Any suggestions would be helpful.