positive OPK Negative Progesterone

I was put on 50mg of clomid for days 3-7. I ovulated on CD 17&18, I know this because of a positive OPK. I went in for my CD 21 progesterone check and my levels were not consistent with ovulation. What is going on here? How come I got a positive OPK if my levels were not high enough. Can I still have conceived ?