Has this happened to you?

I have faithful 31 day cycles (for the last 10 years) I went and took a blood pregnancy test on what I thought was 12DPO. it was negative... that was almost a week ago. My period has NOT started. I called the doctor and he said I may have ovulated later this month due to stress (my husband just left for deployment) so he wants me to wait a week and if i dont start to make an appointment for a sonogram. I have ALWAYS started on CD31 @ 10-11am. I am now ENDING Day 33... so working on 4 days late. I know stress can cause this... but ive been stressed more than I am right now and it has NEVER been late. I took clomid cd2-6 150mg. and I know without a doubt I hit my ovulation cycle bc hubby & i BD'ed every other month all month long before he left. my question is how soon would a blood test be accurate if I THOUGHT I was on 12DPO but obviously I wasn't as my LP is generally 13 days.