Help!

I'm on CD 18 & had clomid 50 mg. This past weekend I started getting faint lines on the OPK but it never got dark enough for a positive. NOW, the line is starting to fade away! I've heard I can ovulate late taking clomid but I'm not sure that's what's happening. Do you think I missed it when I had the faint line? Or am I just over reacting?