I'm guessing you have fairly large breasts because this tends to be a fairly common complaint. When you sweat and it can't dry out quickly, bacteria and/or fungal spores can flourish causing odor and rash. It's the same kind of thing that happens to men when they get jock itch. The trick is to keep as dry as possible.
There's another long thread on the forum - you could probably search 'breast odor' and find it - that has a whole lot of things other women have tried. The only problem is that none of those home remedies seemed to work very well. One trick that I noticed had not been mentioned was laundry starch - the old-fashioned dry powder in the box that you have to mix with water to use on clothing. Most grocery stores still carry a few boxes and they're usually on the bottom shelf.
Anyway, my male neighbor spent a summer in south Texas hanging gutters for a living with his dad and nearly lost his man parts to a horrid fungal infection. He tried all the over the counter stuff and nothing worked. Finally he was so uncomfortable that he mentioned it to his mom. She handed him the box of laundry starch and told him to "pat down" with that every morning. It worked and he swears by it to this day and even uses it if he starts getting athlete's foot. Maybe it'll work on breasts too? Can't hurt to give it a try!
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