Is it a rash or perhaps excema? I thought my son had a rash so we started using baby powder at every change. Baby powder is frowned upon since it can be harmful, but I was desperate. It worked for a bit then stopped. I took baby to the doc and found out its excema (which we knew he had) that's aggravated by the diapers. So basically he'll have it until he's PL'ed. It's not serious and doesn't seem to bother him, but we've been bathing him in Cerave wash and that seems to have helped some.
From my iPhone, probably while BF'ing.