Hi, I can understand your concern for circular bruise on the underside of one of the breasts. There can be multiple possibilities of the circular bruise and it requires a close physical examination to diagnose the exact entity of the problem. In my view correlating with the facts you have provided it appears to be fungal infection of the area. I suggest you to meet a dermatologist to for proper therapy. In between you should avoid sun exposure and friction to the area properly. If I were your dermatologist then presumptively would have preferred to apply clotrimazole or ketoconazole cream. You can take an antihistamine such as loratadine for twice a day if itching is present. Do not try to pinch, pop or squeeze the area as it may cause secondary infection of the area. All the best