It sounds like breast cancer
It can be a cyst, benign or malignant growth. You can get a CT or MRI scan done. However the best test for confirmation of this lump is a FNAB (fine needle aspiration biopsy) of the lump which should be got done after consulting a physician. FNAB is aspiration of the material of the glands which is done by a fine gauge needle (22 or 25 gauge) and a syringe. Then the material is looked under the microscope to confirm the diagnosis.
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.