What Causes Light Spotting During Periods Without Bleeding While On Cerazette?

Hi I usually have a 29-30 day menstrual cycle however I am now 30 days late. I took a pregnancy test about 2 weeks ago and it was negative. My last period wasnt usual for me as it was very light more like spotting. I am on cerezette and have taken this for years but am worried about taking them now.