Negative HPT and AF is late! Help please!?

I took an HPT after my period was one day late and got a BFN. I keep reading and hearing that the test is most likely right because it would be able to pick up the hormone. How true is this? I am now 4 days late but haven't tested again and have no sign of AF. I did have unprotected the whole week I ovulated so I have good reason to believe I may have conceived. Has anyone gotten a BFP after their period was due or know anyone who has? Thanks