Are you using cloth wipes? I thought I was having the same problem, and it turns out it was he wipes. Now I use cloth diapers and disposable wipes, and have no more rash!
The time has come, my online friends, to talk of many things:
Of flats, and wool, and hybrid dipes - of breastfeeding and slings-
And why our bed is cramped at night,
and the joy my munchkin brings!