What Causes Blood Clots Along With Light Vaginal Bleeding While On Birth Control Pills?

Hi, I m on the period part of my first birth control pills since my last baby. I thought I had started my period when I passed a tissue like clot. It was clear with some dark clots. There were some other similar clots too. Very light bleeding since. I had some cramping before It started. I have never seen this type of period before. Could you explane?