I agree that the test may have been bad. Did you ever go back after your miscarriage to make sure that your HCG levels dropped back to zero? On the other hand, I was told that morning urine contains more HCG in early pregnancy than afternoon urine does.
it is possible there was something wrong with the first test and the positive result was wrong. either way i would see a doctor and have them do a blood test. even if you were miscarrying i don't think your hormones would drop that quickly. good luck