Neck rash

Sarah

My LO has had a neck rash for about a month now. It started on the underside of her neck, by the chin, and it was small. Then it grew and then gtransferred to the bottom side of her neck, where it comes and goes now. I have been cleaning her neck at least 2x a day, drying it and putting petroleum jelly. Anyone have any idea what kind of rash this is and how to treat it? Picture somewhat shows it, but it's not the worst part of the rash.