What is the best day of your cycle to get your blood test to check progesterone level while taking Clomid?

Ashley

I went on day 20 of my cycle to get my progesterone level checked to see if I have ovulated. My OB put me on Clomid this month for the first time. My OPK showed a peak at day 19. My test results came back today and my level was 3.5. Could I already have ovulated and the Clomid is making my OPK inaccurate or should I have gone on day 21-27?