I used to have ths with Lucas when we used the lovebums and stacinator PUL wraps. He seemed to have some reaction to the fleece, it started with his belly button and then he got really bad blistery welts on his thighs too
It seemed to be where the pul is stitched to the fleece cuffs, I assumed it was trapping moisture there...
Maybe try a barrier cream around her belly button? vaseline or something