I have this big lump under my armpit and i would like to know what it is how it got there and what i can do to treat it ?

Unsure: The lump needs to be examined by a physician and considered in the context of any other possible symptoms. Common causes include lymph node enlargement (can be associated with simple infection such as a cold or mono, or can be serious such as an early sign of cancer). Biopsy of an enlarged lymph node may be considered. Cyst, fat deposit, inflamed hair follicle are some other possibilities.