HPT vs. Doctors Office Pregnancy Tests.....

False + hpts are very rare.. sometimes if one observes a + after about 10min, this can be an evaporation line

Did you take a urine or a blood test at the health department? You might have been using a more sensitive test than they were. I would go get a blood test. That will be accurate.

As Dr Q said, false +ives are rare, assuming you read the test in the correct time frame (under 10 min). You could have had a chemical pregnancy, which would explain the discrepancy, but best if you just get a blood test to give you answers.