Hello, I Am Taking The Nexplanon Birth Control Implant, And

Hello, I am taking the Nexplanon birth control implant, and I have bee on it since March, 2015. Ever since I got it, I have had very consistently, regularly scheduled periods within reason. (It is an odd schedule for days in between: teens, twenties, teen, twenties, etc.) For example, 16, 25, 14, 28, etc. Instead of every 28 days. I was fine with it because although it was unconventional, I knew when it was coming.

Well, my last period began on September 21 and ended on September 26. The following period was supposed to be a teen one according to my calendar. It still hasn't come... I took a pregnancy test last week, and it was negative. I took another one today with the same negative result. At which point should I be concerned and go to a doctor? (And should I go to a family doctor or a gynecologist? I am not a patient of either...)