Thank you for the query.
is most likely caused by hair follicles bacterial infection. Hair follicles are getting obstructed and result with such image. I suggest you to try wash it twice a day with soap and water and try to keep it clean and dry. You should also keep it as much on the air as possible (try to keep your arm up, so the armpit stays open). If this wont help, and you will find it getting worse, you should visit your doctor and get antibiotics prescribed.
Hope this will help. Feel free to ask further questions.