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 and sneaky.
I once got a rash all over my chest and both breasts while on a trip. I was nursing an 18-month old at the time.
Never did figure out the cause, 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