Could it really be?

Well it is possible for a tubal ligation to not be effective, one of the tubes can still produce an egg which can find its way to the uterus, however chances are this has not happened as it is extremely rare... But to be on the safe side I would make a doctors appointment and take another test in the mean time to settle your mind a bit, sometimes our cycle can play mind games with us, and sometimes there are things going on that only a doctor can understand and figure out. Best of Luck!!