OPK's and HCG?

I received a couple free opk in the mail when I ordered my pregnancy tests. I've never taken them before so I decided to try one. Some of you may have read my other post, I'm two days late (I'm never late.) And the hpt is coming out negative from what I can tell but my question is would there be a line on my OPK if I was supposed to be on my period? I didn't think that made any sense but there is a line showing up on that one.