I Thought I Might Be Pregnant So I Went To

I thought i might be pregnant so I went to a clinic and the test was negative however the doctor said it wasn't 100% due to it only bein 2 and a half weeks since I last had unprotected sex, she then said I could start taking cerazette if I didn't want to carry on with the pregnancy if I was and she told me that my periods would stop however I had missed a month anyway but 6 days after I took the pill I have had heavy bleeding and clots what could this be?