The pain in the armpit may be caused by the following reasons: 1. It is possible that the sebaceous cysts in the armpit are inflammatory and cause localized pain, and a granular, hard swelling can be felt in the armpit. The swelling of the lymph nodes in the armpit is usually caused by two reasons: infection of the upper limbs or soft tissues around the armpit, or malignant tumors in the breast or chest that metastasize through lymph. 3. pain.