Possible stitch left in upper outer eyelid

Were the stitches dissolvable? If so it will eventually dissolve.  A nodule can be from a suture, and can be from a buried suture if those were also used.  It can also just be a type of cyst, or an inflammatory nodule.  May need to be either excised or injected.  Sometimes it hurts to remove sutures. 

your best option would be to have your surgeon evaluate your scar. if there is a remaining stitch, it can be removed. If there is a different issue, it can be treated.

Every surgeon has slightly different preferences in terms of surgical techniques, and details like suture types and duration of sutures; having said that, most eyelid surgeons will remove eyelid sutures before the end of the first post-operative week, for fear of leaving visible suture marks.  While eyelid incisions tend to be very forgiving due to the thin, delicate eyelid skin, a bump or two of scar tissue is not unusual in the first few months of healing; this can be easily and well-treated with a tiny injection of dilute, anti-inflammatory steroid.  If a retained suture is indeed the cause of the bump, removing it may very well resolve the bump without a big difference in the final scar.  Without an exam, it's impossible to comment on your specific situation; you would be best served by following up with your surgeon.  Good luck in your recovery!