Hi Ladies

I read a lot of stuff on the forums and never post much, but needed some opinions. Yesterday, I had a little bit of pink discharge after a BM and then a couple hours later, mild period like cramping. About 2 hours later I had a little bit of bright-darker red spotting. It's day 15 of my cycle and about the time I "should" be ovulating. (i also had a headache all evening too) I took an OPK last night and this morning and they were both negative.
My question I guess is, when should I have gotten a positive OPK? The day before yesterday, possibly tomorrow? Or is it possible that my body tried to ovulate but was unsuccessful despite the spotting???