Rash under breastshelp me?

Eczema. It could be eczema or warts, or an allergic reaction to something. Rashes are best treated in person so the doctor can see and feel to determine what it is.
Candida. The most likely cause is candida (a yeast). Try an otc antifungal cream, and keep the area as dry as possible with talcum powder. If that doesn't work, you'll need to see a dermatologist, but it probably will.