A painful lump in armpit that comes and goes can be a swollen lymphnode, in response to fungal infection or sweat dermatitis or infection of hair follicles. It can also be an enlarged lymphnode due to malignancy of breast (rare in males but not impossible). At times it is a cyst too and this can get infected. Lipoma is also a possibility. Consult your PCP for an evaluation. A CT scan can be confirmatory or a fine needle biopsy. Treatment can vary from wait and watch to anti-inflammatory along with antibiotics to excision of the lump. Check for diabetes too as high sugar often results in repeated infection and boils. Do discuss this with your doctor and get yourself examined. Take care!