Lumps may occur when lymph nodes are swollen. Lymph nodes are filters that can catch infectious organisms or cancerous tumor cells. When they do, lymph nodes increase in size and are easily felt.
Axillary abscess like this usually caused by bacterial infection, cancer, fungal, vaccination, viral infection, and so on. So, a breast self-examination should always include the armpit because breast cancer can enlarge the armpit lymph nodes even when the breast itself seems perfectly normal.
If there’s something suspicious, contact the doctor nearest you.