B.M.
hey P., i have a 13 mos. old baby boy and he developed a very similar rash - it was only on his abdomen and back. i couldn't figure out what it was for the life of me! and like your baby, he didn't seem bothered by it at all. finally, his dad told me that over the weekend he bathed our son with Dial. well that explained it for me! at the pharmacist's suggestion, i used cortizone cream .5% and it cleared up in two days. in the interim, i quickly switched him back to baby soap. i don't know when to switch to adult soap but i know now's not the time! i'm sure that's exactly what happened w/your baby. :)
Good luck! :)