Hello! I have been through your question. Related your concern I think this could be a folliculitis which is an inflammation of hair follicles. Usually you can have folliculitis on any part of your body that has hair but is most common in some areas like armpits as you have, or legs or buttocks and can be caused by bacteria and rarely by fungus.It is a good news that it doesn t hurt and you have drain out the pus. You should know that most of the time, this kind of folliculitis goes away on its own in 7 to 10 days but in your case I suggest to use local antiseptic (betadine) after the drainage. Yes of course, you should keep a closer eye on it, if you have mild fever and have pain, and notice reddish swollen area in this place you should meet a doctor for further physical examination and lab test and to start an antibiotic (oral or local) if need it. I hope my answer help you. I wish you a quick recovery.