Visit a doctor
A lump on your armpit could be a lot of things: like your body's reaction to an infection (your lymph nodes will swell up), a bug bite or something more.
Risk factors for this being something serious are: if it is only on one side; if you don't have an infection; you don't remember any trauma there.
Visit a doctor and be prepared to answer questions about family history of major illnesses.
Answer assisted by Tejas Ozarkar, Medical Student