If you already missed your period in August then you should be able to get a reliable result from a pregnancy test.
Chances are that you waited too long to read the results on the pregnancy test and saw what was infact an evaporation line. Or you were pregnancy and had a very early miscarriage.
I have seen people go to clinics though and get false negatives. Maybe because they drank too much water before they went in. What I would suggest is making an appointment with your doctor and getting some blood work done.
