I was diagnosed with invasive breast cancer in Aug. That is exactly what mine was a sore lump under my armpit on the side of my breast. My sis and I went for mams and mine was cancer.
How old are you?
Do you have other signs and symptoms?
Can you feel any lumps in the breast?
A swollen lymph node in the axilla can be caused by several conditions. Breast cancer is just one of them. Infection near the site of the lymph node can also cause enlargement. Lymphomas can also present with enlarged lymph nodes.
It would be best to have yourself seen and examined by your doctor.