I agree with Joyce, you should go get bloodwork done. False negatives are common, false positives are not. However, if it was that faint it could also have been an evaporation line or as Joyce said, a chemical pregnancy. I truly hope it turns out to be a positive, but see a Dr. to put your mind at ease. Best of luck to you!!
i would get a appt with a doc to have a blood beta done, to check levels....sounds like you are, was the negitive done correctly, could you have maybe not had enough urine on it? it could be a early miscarriage or a chem pregnancy.....if you are concerned go to doctor or ER