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The bottom side of the breast is a dark, warm place that is prone to heat and fungal rashes, so that is the most likely explanation.  If this rash has lasted longer than a week or if it is really uncomfortable, I'd suggest you show it to a doctor.  You would need a different type of cream for a fungal rash than for one from another cause, and the doctor is the one who can tell you what kind of rash you have.  There is a type of breast cancer called inflammatory breast cancer that presents with red, dimpled skin. It is usually accompanied by swelling, pain and itching.  So if your symptoms get worse or don't go away, make getting to a doctor a top priority.