WHAT ELSE COULD IT BE?

I HADN'T HAD A PERIOD SINCE  JULY 21,2013-SO, I TOOK A PREGNANCY TEST ON AUGUST 1ST,2013 AND IT CAME BACK NEGATIVE-I HAD MY THYROID REMOVED IN MARCH OF 2013 SO, I CHALKED IT UP TO THE CHANGE IN MY BODY DUE TO MY MEDICATION-SO I TOOK 2 MORE PREGNANCY TESTS AND THEY CAME BACK WITH A FAINT POSITIVE-SO,I WENT AND TOOK A BLOOD TEST AND IT CAME BACK NEGATIVE.I HAD MY TUBES TIED 11 YEARS AGO-AND MY PERIOD STILL HASN'T COME.SO, ON OCTOBER 1ST,2013 I STARTED SPOTTING-WENT TO THE HOSPITAL ON OCTOBER 3RD,2013 AND THEY TOOK BLOOD AS WELL AS URINE BOTH CAME OUT NEGATIVE.THEY WENT ON AHEAD AND DID A SONOGRAM AND A VAGINAL SCAN-DIDN'T FIND A FETAL POLE OR YOLK SAC BUT, THE ULTRASOUND SHOWED A 6MM ENDOMETRIAL CANAL FREE FLUID OR ELONGATED TUBULAR GESTATIONAL SAC-THE DR. SAID THAT IT WAS EITHER THE ENDING OF A PREGNANCY OR THE START OF 1-I HAD SOME SPOTTING BUT, NEVER HAD ANY CLOTS-WHAT COULD IT POSSIBLY BE? PLEASE HELP ME...THANKS