What Causes Light Vaginal Spotting While On Primolut?

hi ..I take a primolut-n because my period continued approximately 30 day with heavy bleeding last 10 days ..tis quit with first 2 days (with primolut) ..till now I take 17 tablet but today I sow a light blood spots ..why? there are spots when I not finished the whole packet. the procedure was 3 tablet till the bleeding stop then two for 3 days then one till finish the packet. my best regards... lavender