It could also be thrush (yeast infection)
I should also mention I get that too and it's from my breasts growing so quickly in pregnancy and I can no longer wire underwire. I found only breathable cotton maternity bras are comfortable and keep me rash free I got the sleeping maternity bras for breastfeeding from motherhood maternity online. Right now they are buy three get one free
If you go to mayoclinic's website they have pictures of all sorts of rashes so you can look at your rash and then look at the pics and see which one matches.