unexplained af with preseed

Hi all, can anyone explain whats happening to my body.i have taken pre seed for 2nd cycle and started brown spotting on day af was suppose to arrive..the brown spotting continued for 1 week and i had heavy af flow with big blood clots until 3rd day of af.the clots are big and long size.no bfp and did an ultrasound..not pregnant.so what made this?? Anyone experience it..