As you are a lactating mother, chances of any collection including breast abscess etc are high.. chances of malignancy is remote per se . Best is to consult your gynaecologist or a surgeon nearby . An ultrasonography will definitely help u in this.. it will show whether its a mass or collection or mammary duct dilatation etc
Next Steps
ultrasonography
consultation and examination by a surgeon
Health Tips
Dont think of cancer as primary cause..it can be a remote possibility in current scenario.. my wishes for qyick recovery