I had a similar experience with my first 2 pregnancies. Wait a few days and if AF doesn't appear as normal then test again, it may just be that you are testing too early. It took 3 tests with a couple of days inbetween before I got a BFP my first 2 pregnancies. Although the tests are very sensitive there are a lot of factors which can vary their accuracy.

Good luck, hope you get good news soon