Hello. I am Dr Adil Thank you for your question. Lump under armpit is usually an enlarged lymph node. But i would also like to know where it is? is it present deep inside the skin or over the skin, is it mobile (movable), is it soft or hard, Does it appear suddenly or it has been there for a long time etc. As you have said that you have noticed the lump suddenly and it is also painful, it doesn t look like cancerous. It looks like enlarged secondary to some infection. I would also like to know, whether do you have any lump or pain in your breasts? Because breast infection may also cause enlarged axillary lymph nodes. It could be a hair follicle infection. Any how i would like to examine it and ask more question to understand it completely. In the mean time, you may discuss with your doctor to start some antibiotics and anti-inflammatory drugs. FNAC and biopsy can be done to rule out dangerous causes. Kind Regards. Dr Adil