fungal rash on baby

Hello Dr Mom, my little boy has developed a rash around his groin area that my pediatrician said is not a diaper rash but fungal. I am just wondering where that comes from? Is it from me and breast milk or the antibiotics he had at birth for a few days. I have a cream to put on it but wondered what else I can do. Should I be taking probiotics or anything else to help him out. What else could I put on the rash to help heal and soothe it? Thanks, Samantha