What Causes Abnormal Menstrual Discharge While Being On Birth Control Pills?

Hi. So I have been having heavy and irregular periods all my life. Last 2 months I was on birth control pills yasmin and mercilon. With yasmin my side effects were bad so I switched to mercilon which I tolerated well. My biggest fear is I had this excruciating stabbing cramping pain on the 1st day of period. And I pass out this big chunk of greyish clot I guess with some blood. Once this was out my pain immediately stopped . I had my heavy flow after that. My cramps are usually bad on the first day. This only happened to me twice while I was on the pill. What really happened ?