What Causes Delay In Periods?

I had a miscarriage Feb 22-2011 (bled for about 1-1.5 weeks) (fetus exited while going to the washroom). Ultrasound was completed a few weeks later and NO D&C required. I had my first period on March 30-2011 .. normal period for me ... 5 days total. I have not had a period since .. I'm on CD 39. (before miscarriage I usually cycled between 32-34 days) I have no idea either when/if I ovulated. Took 3 hpt's and all came back negative. Is it possible the hcg is still not high enough to detect if i am pregnant?