Just a long shot but... for those who breastfed/are breastfeeding, did you ever notice you had a rash on your breasts too?
Mine looks/feels like eczema (I get eczema anyway but never on my breasts until now). It responds a bit to hydrocortisone but not to e.g. Daktarin. I've had thrush so I know what that looks/feels like, and this is at the top of the breasts anyway, nowhere near the nipple. It started when I started BFing though thinking back it may have been there in later pregnancy too, and seems to get worse the more I BF (have had supply issues - long story).
I'm on a dairy elimination diet right now to try and help my DS who has all the "usual" symptoms of dairy allergy/intolerance. I'm wondering if perhaps we're sensitive to the same thing, and now that my breasts have "matured" the offending substance is in the milk ducts and somehow causing the rash there.