This could be a skin problem or a bruise.  I'm not clear whether the place is under your breast on your midriff or on the underside of your breast and actually affecting the breast itself.  Either way that area is a warm, dark, moist place that can lead to various types of rashes or even blisters if there is a lot of friction when exercising.  If this doesn't heal within a week, you should call the doctor to get it correctly diagnosed and treated.  If you run a fever or if the purple area gets larger, call the doctor right away.  You might have an infection that needs an antibiotic.  This is not likely to be cancer, but it may well need a doctor's expertise to determine the cause and best treatment.