The first possibility that is coming to my mind is mammary duct ectasia.It causes one or more of the ducts beneath your nipple to become inflamed and clogged with a thick, sticky substance that's green or black. You can feel relieved by applying warm compresses to your breast. Taking aspirin or nonsteroidal anti-inflammatory drugs, such as ibuprofen, may help relieve some symptoms. Also consult a gynecologist as antibiotics may be needed.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.