I would get a bleed sample analyzed and make darn sure it is not MSRA, the antibiotic resistant , deadly strain of staph. especially , if you have, or know anyone who works in or around hospitals or visited one recently
An abscess is an infection and could possibly be from an infected hair folicule depending on the exact location of the abscess. Friction from your arm against your side could also be causing the abscess. It needs to be opened, drained and cleaned. You might also need antibiotics to prevent further infection.
Underarm perspiration is blocked due to blockage of sweat glands situated there. This blockage causes abscess. Do not use antiperspirant sticks. They block sweat glands and cause infetion like abscess.
Usally the cause for this is an infected hair folicle under the skin. However, if it just feels like a lump under your skin you will need to see your doctor because it could be a swollen gland signaling somthing is wrong with you.