What Causes Early Prolonged Periods?

I GOT MY PERIOD A WEEK EARLY AND NOW I HAVE BEEN BLEEDING FOR OVER 11 DAYS . IT IS LIGHT AND VARIES FROM BROWN TO RED . I WENT TO THE HOSPITAL MY BLOODWORK WAS NORMAL AND I AM NOT PREGNANT . WHAT IS HAPPENING I AM ONLY 35 . IM VERY SCARED AND NOW I WAS TOLD TO TAKE THE PILL TO STOP THE BLEEDING . HOWEVER THAT WILL NOT TELL ME THE UNDERLYING PROBLEM ?