Ovulating??

I've never ovulated that I know of. I'm on my second round of Clomid cd13. I've been taking two opk's a day, but have yet to get a positive. However they have been darker than ever before but still not positive. Starting today I've been cramping pretty bad on my left side only, what feels like my ovary. So my question is how long after you feel the cramping do you normally ovulate, or is it happening now and I'm just not getting a positive for some reason? Thanks