If it is fungal like athlete's foot, I have a cure that will sound really crazy, but try it. I know people who have used it to cure athlete's foot and I have used to cure a fungal infection on the skin as well. It is urine, yes, I said it, urine. Just pee in a cup in the morning and set it to the side for the day, preferably where no one will notice it and you can dip cotton balls in it and apply liberally under your breasts. I would do it 3-5 times per day until the rash is gone. It should only take a few days.
If it is yeast, I would eliminate all sugars, dairy and flour products from your diet. Take lots of garlic tablets or eat raw garlic. You could also mince garlic and put it in some olive oil and let it absorb the garlic, then apply to area. I think that Miracle gel that someone else suggested would work as well. I know a Naurapath that sells it, ####-###-#### in Safety Harbor. She also sells anti-yeast sprays and supplements.
Here's to your quick recovery!