This is actually a common problem, and one that is easy to treat. This condition is called intertrigo, a skin infection from yeast. It occurs under the breasts (and other
skin folds), since yeast love to live where it is moist and warm.
To help the problem, you should make sure that you cleanse the area daily and dry the skin thoroughly before putting on your bra or other clothes. You can use an over the counter anti-fungal cream (like clotrimazole) for a week or two to help get rid of the rash. Once the rash is gone, continue to keep the area clean and dry, as the rash can easily return. You can apply cornstarch powder under the breasts, which can help absorb excess moisture to help keep the rash away.
Regarding your comments about breast cancer, the first symptom is usually a firm lump that you would feel in the breast tissue. Therefore, you should check yourself monthly with a breast self exam and get mammograms yearly to reduce your risk.