Can you still be pregnant if the blood test was negative?

I am usually as regular as clockwork with my periods. I have been on Clomid for 2 months now. My last period was 3 Dec 2008, my cycle is 27 days. I am 7 days late. I had a blood test today and it was negative. I have so many pregnancy symptoms though. Could I still be pregnant and the test could not pick it up because the hcg was to low?