What Causes Vaginal Bleeding Continuously?

I HAVE BEEN BLEEDING NONSTOP SINCE JAN 2010 DOCS PUT ME ON DEPO SHOT THAT DID NOT WORK SO TRIED NORA B BIRTH CONTROL NOTHING WORKS THEY HAVE NON ULTRA SOUNDS XRAYS AND CAN FIND NOTHING WRONG I HAVE BEEN BLEEDING 6 MONTHS FROM LIGHT TO MODERATE TO HEAVY WHAT VOULD BE WRONG