The underside of the breasts is a warm, damp place susceptible to several types of rashes such as fungal rashes, heat rash, and allergic reactions. If it is the same on both sides, it is probably not cancer-related. However, knowing the best treatment depends on an accurate diagnosis. If you recently changed laundry detergents or skin care products, you might drop the new product and see if you improve without treatment. However, if this rash has persisted despite home remedies, it's time to see the doctor. Using the wrong cream or ointment could actually worsen your condition.