What Causes Back Pain, Lower Left Abdominal Pain, Heavy And Prolonged Period?

Hi I m 39 and have had heavy clotty periods for along time.. Recently I had my period which usually last 9-15 days .. And I finished and started again 3 days after with cramps and a release of clots and back pain.. What could this be.. I m not on any birth control... And I ve noticed I have a pain on my left lower abdomin... Thanks