Your doctor was NOT much of a help here, but they could have been.
A quantitative blood test will tell the level of hCG in your blood. In a few days to a week you would then go back and have another test. If the level is rising, as it should, you are pregnant. If it drops, or isn't rising properly, there may be a problem.
The quantitative test tells you the specific amount of hormone. The OTC urine tests are a "qualitative" type of test - are you or aren't you.
Since it can take a few days or longer for the levels to drop low enough in your urine you'd still test positive even after a miscarriage.
Hang in there. Some bleeding is normal early in pregnancy.