What would the lump in my armpit be?

Probable abscess.: Probable abscess. Apply moist heat with a towel or moist heating pad and it should go away. If it gets red, bigger, you have a fever, see your doctor for antibiotics or possible i and d ie. Incision and drainage.

Axillary lymph node: Enlaged lymph node can be due to an infection in arm or hand on that side. Could be from something like monomucleosis, in which case oth nodes would be enlarged ; you would feel genaerally unwell. Possible malignancy such as breast cancer or lymphoma, is always a possibility. Localised abscess is possible, this would be tender and red ; deserves a medical visit ; antibiotics.