Heat, sweat and friction which bras aggravate are a trigger. When broken out even pressure without friction seems to aggravate it. I have a double whammy as I have an allergy to elastic, Mercaptobenzothiazole, which is an element in rubber and used in blended material and of course elastic is in most bras. I wear Cottonique 100% cotton bras, or the bra liner which does have a band of elastic on the bottom which I remove and replace with a drawstring. Victoria Secret their bras are not!