you must have ovulated later in your cycle - and your period is not due yet.
or you could be pg, but for the same reason test won't show it yet as you are less then 2 weeks pregnant IYKWIM
happened to us, DH had a lovely Friday evening, thinking better today as period's due tomorrow, but I got pregnant instead as my ovulation was delayed by 2 weeks (due to illness/stress/run down body etc).
therefore even being 2 weeks late test was still negative!!!!
I had a positive result only 3 weeks after missed period and was definitely pregnant - my beautiful blonde DS2 is now 9 years old!
so there's hope yet - in fact if you've been ill or any other things may have caused a delay in your ovulation you could even be ovulating now!
you could test that and TTC?
best thing is not to obsess, I'd take 1 pregnancy test every 3 days only until either positive result shows or you have a period.
(compared to 9 months pregnancy another 2+ weeks of patience is child play! and good practice!!!