Is it painful?Has it been there awhile or is it a sudden development?Then it could be folliculitis which is quite commonly seen in the armpit.
Another cause if the lump is small to medium sized could be a lipoma or enlrged lymph nodes.
Sometime shormonal changes in a normal menstrual cycle will cause these lumps which should settle down as your cycle evolves.
A lipoma is a benign tumor composed of fatty tissue. These are the most common form of soft tissue tumor.Usually, treatment of a lipoma is not necessary, unless the tumor becomes painful or restricts movement.Lipomas are normally removed by simple excision.
Folliculitis is the inflammation of one or more hair follicles.Folliculitis starts when hair follicles are damaged by friction from clothing, blockage of the follicle, shaving.In most cases of folliculitis, the damaged follicles are then infected with the bacteria Staphylococcus (staph).
Topical antibiotics such as mupirocin ointment can be applied on it.
I've noticed a small pea sized lump under my left armpit for years. I often would cut myself over it while shaving. Today it was itching and figured it was an ingrown hair. I started squeezing around it much like a pimple and after a pretty strong try a small white pebble-like object came out! Is this deodorant build up or something more serious? The pressure feels so much better and there is no pain or bleeding. Should I be concerned?