A lump in armpit 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. It can be a boil or infection of hair follicle. 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. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.