Some ideas, in no particular order:
1. bacterial infection
2. allergy to something the baby's eating or taking
3. poison ivy
4. fiberglass particles (attic insulation?)
5. new laundry detergent
6. new underarm deodorant
7. fabric in bra is irritating (new bra?)
8. recently took up swimming - chlorine sensitivity
9. Reconsider thrush. It can be persistent.
I once got a rash all over my chest and both breasts while on a trip, was
nursing an 18-month old. Never did figure it out, but a topical steroid
ointment cleared it up in a few days.
Linda J. Smith, BSE, FACCE, IBCLC
Bright Future Lactation Resource Centre
Dayton, OH USA
http://www.bflrc.com