regular periods* 6 days late with BFN

Cassidy
I have 21 days in my cycle and my periods are very regular and tracked however I haven't had a positive pregnancy test. I've tried pink dye, blue dye and digital and they've all been negative. I am not on birth control and had unprotected sex on august 21st.. the confusion is killing me.. is it possible my HCG levels are too low for detection at the time?