That is an excellent question and one that is best answered by your primary care doctor
or another medical care provider who can perform a complete history and physical examination and decide upon whether further testing is required to evaluate your lump in the armpit. There are several potential causes for what this could be but unfortunately, based upon the minimal history provided, it is difficult to know the exact cause but some potential causes will be discussed here. Only your doctor can evaluate you and decide upon the cause of this and whether further treatment is indicated. One potential cause could be an infection in the hair follicles of your armpit, this is sometimes called folliculitis and can sometimes progress to a worsened infection. Another potential cause of this could be am enlarged lymph node which could be related to multiple causes including infections. It is very important that you are evaluated because based upon your age and other comorbidities, this could be an early sign of something more concerning and so only your doctor can decide this. They may want blood tests or further imaging studies to evaluate this lump, and it may even need a needle aspiration to see what is inside.