What Causes Cramping And Heavy Vaginal Bleeding?

I started. My period Friday night. I started spotting. Like a dull brown. Then didn't start really bleeding till early am. Haven't been heavy. Then now tonight I went to the bathroom and wiped and there was a huge bright red and dark clot.it was bubble or balled like bit it was a full clot. There was two.and I just passed another dark one. .I have slight cramping.when this happened the bleeding was heavy. Now its not as bad ..could this of been an embryo misscarriage? ??