late period but BFN?

My period is 4 days late now, I'm 16dpo and I've tested with fmu for the past few days (with various tests, First Signal, First Response, and Clearblue) and all are negative, not even a faint line, but still no sign of my period. I've been tracking my cycles for about 6 months now; My periods are pretty regular, never more than a day late. My bbt is still going up as well. Should I consult a doctor? Is it possible that implantation occurred late and BFP could still be right around the corner? This all just seems odd to me.