A hard lump in the armpit found while breastfeeding may be due to acute mastitis or parametrial hyperplasia.
Acute mastitis often occurs about 3 weeks after delivery and is caused by milk stagnation and bacterial infection, often manifesting as breast pain, redness, swelling and fever, often accompanied by swollen lymph nodes; parametrial hyperplasia, on the other hand, presents only with lumps, mostly without severe pain or systemic symptoms, and with only mild swelling and discomfort.