What Causes Heavy Bleeding And Clotting With Stomach Cramps Mid Cycle?

Hi, I have recently had my period which lasted for a length of seven days which is normal but 4 days before I actually started my period I bled heavily and then stopped the same day. My period was normal for the seven days but I finished it five days ago and today it started again very very heavy and clotty. I have tummy cramps and have taken a pregnancy test which was negative. I have been to a GP but they were unable to give me a conclusive answer.