What Causes Absence Of Periods While On Nexplanon Birth Control Pills?

Hi I am currently on Nexplanon birth control and it is due to be replaced in March. Whilst being on the birth control I have not had any periods. However, recently I have been having brown stringy discharge around the same time for the past two months. Also been feeling run down and tired aswell as frequent headaches. Is this normal or should I be concerned. I am 21 years old.