It is probably a topical yeast infection, don't use cornstarch or baby powder, they will hold moisture against your skin and make it worse. Go to a pharmacy and talk to the pharmacist, there is a cream they can give you for this with out a prescription. It is basically the same cream that is used for jock itch and it really does work to make the rash go away.
If your bras have particular types of plastic or rubber in the part that goes under your breasts, you may have developed an allergy. As an experiment, switch to all 100% cotton and wear the bras loose. Talk to the pharmacist too.
I have unfortunately first time besides being pregnant have gained slot of weight for my size and some of it is in my breasts, I have had that problem once or twice this year (I shower everyday, twice sometimes). Like the other person said Vaseline is an amazing product, it was the only thing I used on my 2 children when babies if a diaper rash appeared. It really does work!