L.B.
Well my first response is what kind of detergent do you wash with? Could it be an irritant? I myself only wash with Free & Clear products and unscented fabric softner.
The other thing that comes to mind is that perhaps it is a fungal infection. We've all heard of kids getting thrush and mom's getting it on their breast from breastfeeding...maybe this is similar? My BFF would put some anti-fungal cream on her breasts when this would happen and it would clear it up...maybe an idea?
Considering the weather and time of year, it could also be dry skin. Do you have a humidifier in her room? Do you put lotion on her immediately following a bath?
I would certainly discuss this with your peditrician...if you can't get her in, I would at least call and talk to his nurse and see what recommendations she has.