I have a very painful lump in my armpit?

Cyst?: There are few possibilties. See a dermatologist to examine you and recommend treatment.

Arm pit lump: Most likely would be an inflammed sweat gland. Probably its infected. For treatment: use a topical antibiotic and do not use anti-perspirant type deoderants. Often these come after shaving the axilla (arm-pit), so wait til it is healed. You may need an oral antibiotic if a topical does not work.