same thing happened to me, AF was due on a tues i did a test, well i did a couple of tests and got BFP, i went to the docs a few days later all excited and got a BFN, i was so upset, he gave me a few tests to do myself at home, i think the CB tests were actually more acurate than the docs.
i tested every day after and got a a few more BFN, i tried to put it out of my mind, wasn't easy, and after a week and about 13 tests i got some consistent BFP and i went to the docs and got a BFP, they said i was prob because my HCG levels weren't strong enough because it was so early in pregnancy even though i used my first wee every morning.
so all i can say is best of luck, try to take your mind of it for a few days if possible and i'll keep my fingers crossed for you.