Bleeding after Depo-Provera

I have been taking the Depo-Provera shot off an on for 17 years with little to no problems.  My period was non-existent during these times and would generally come back after a year.  I took two years off from the shot as I was having problems with depression and weight gain.  I got a shot in December 2011 and two weeks after my period started and has not stopped.  (It's October 2012 now).  My doctor has put me on three different birth control pills to try to stop the bleeding, however, as soon as I stop or miss a day the bleeding resumes.  Also, it is much heavier than usual and has clotting.  I have had tests run, including a transvaginal and have had nothing else show up.  I'm am extremely worried at this point and would greatly appreciate any recommendations or common stories.