Suggest Treatment For Swollen And Black Area Around Nipples

Hi doctor. I am 22 years old and have never had any problem with my breasts until recently one of the many 'grooves or round surfaces on the nipple' became swollen and sore. I thought it was a result of my sexual activity and didn't care much until the pain persisted. I applied some antibiotics a few days ago and just now I had it checked to realize to my horror that the swollen part has now become black! What is happening to me? Thank you!!