C.M.
I had that problem, too and the doctor told me to test the second morning urine, not as soon as you get up in the morning, as it can give a false positive with some people. I also went to a fertility clinic and the doctor there told me to test between 12 p.m. and 3 p.m. for best results. Also, yes, once you ovulate the egg is only fertile for 24 - 48 hours. So, if you want to get pregnant, after you get a positive result have sex the next day, not the same day. However, look up "natural family planning" or Billingsley Method. Your cervical mucus is the best indicator of your most fertile days. Good luck!