Many women do sweat under their breasts - usually women with bigger busts are most at risk. Due to the lack of air circulation, your skin will sweat there; and, the sweat will be unable to evaporate. This can cause a rash or fungal infection that smells. There are several ways to cure this naturally. As LPogue states you can use vinegar in the shower. This works well.
I prefer to use lavender oil, tea tree oil or comfrey gel. I dry the area under my breast thoroughly making sure I have removed all dead skin. Then I apply a small amount of lavender, tea tree or comfrey on the area massaging in gently. Then I apply some baby powder (made with corn starch not talc) over the entire area.
Works for me.