Hi swtt
Most likely it is from yeast. You can get some cream over the counter to use on this. Gold Bond Medicated Powder is another good product.
If you sweat a lot under your breasts, it is the perfect breeding ground for yeast. You might want to make sure you are wearing a cotton bra, or keep some gauze between the skin when you aren't wearing a bra.
Hope that helps,
Kerry
Kerry, RN
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