When to blood test for progesterone?

I got a positive opk last Monday night and Tuesday am. My obgyn said to come in for blood test to check progesterone once I ovulated. I went in Wednesday morning and the blood results came back low at 1.8 - she said I didn't ovulate. Did I just go in too early for blood work? I have long cycles (35-40 days) and went in on CD28. I got positive opk on CD27. Thanks for any help!