Why Am I Having Light Bleeding With Minor Cramps After Taking BC?

I am takingYaz birth control for the past year. I took the last active tablet on 4 March. My period only started on 9 March (the day I took the first active tablet for the next cycle.) I've had light bleeding since then (10 days now) with minor cramps and a feeling of fullness. I took 2 home pregnancy tests already, both negative results. What can the problem be?