Ovulating on a natural cycle?

I recently completed my third cycle with clomid, and it was unsuccessful. I've never gotten a positive opk, ever. Well, I've been referred to an RE, and while waiting on my appt (Jan 26) I've decided to not take clomid and try to relax and enjoy the holiday season. CD 17 and on a whim decided to do two different brands of opks. The wondfo came out negative, but the target brand was positive! Totally baffled by this! Has anyone been unsuccessful on Clomid but then ovulated on a natural cycle after clomid? I'm not sure that I trust it, but BD just in case!