It could be late implantation resulting in a late AF and a later date for the HCG hormone to show up in your urine (it starts to be secreated a few days after implantation). Also what test were you using? Something like Clearblue tests at a level of around 50miu and that's quite high given some early tests (cheepies off ebay are great!) can test at 10miu and give a much earlier indication. Book a GP appointment and have a blood test done, that's far, far more accurate than a home urine test.