I'd guess dairy sensitivity. Did you nurse the first two months and then switch to formula maybe?
Is the rash weeping? Like broken skin etc? If it is it's eczema.
How about the laundry detergent? Dreft or all free and clear should be good to use.
My son had eczema. I used eczema lotion I think it was aveno along with eczema otemeal baths. It's a powder that comes in little packets you put in the bath water. I also used dreft baby detergent and organic sent free soap for the bath. Also don't bathe baby too often that will dry the skin out even more.