What Causes Dark Brown Vaginal Spotting While On Cerazette?

Hi there. I started taking Cerazette 6 months ago. I had two regular periods then they stopped for a few months. About 8 days ago I started a period. It went away for a day and now, for the past few days, it has come back but this time it's the dark brown spotting that you would usually get at the start or end of a normal period. Is this normal and how long should I wait until I go to the doctor to see if there's something else that's wrong. (P.S. I'm very good at taking my pill at the right time and have not forgotten any)