It is difficult to tell from the photo. It could be a suture a deep suture that is coming to the surface or it may be a seroma. Either of these are fairly common after an arm lift. Make another appointment with your plastic surgeon and they can evaluate the area. If it is a suture this can be easily removed in the office. If it is a seroma you can aspirate the area with a small needle. Occasionally you have to open the area and allow it to heal in secondarily. Good Luck!
Timothy A. Janiga MD, FACS
Photo not helpful....
Thanks for your question. Unfortunately, based on your description this could be a suture granuloma, seroma and/or inflammation. You should request to see the physician for an examination of the area and a diagnosis. Best wishes always, Dr. ALDO