Ovulation bleeding with no positive OPK

I'm having a crappy cycle this month! I usually get a positive opk on CD13 and have been having ovulation bleeding around CD16. However this month I've had no positive opk having tested on days 11, 12, 13, 14 but on CD 14 and 15 (today) have bright red spotting. So maybe I have ovulated early or not at all. Not noticed change in CM and no sore boobs, which I usually get around ovulation. Am wondering whether to just stop using OPKs or to try Clear Blue, but they're so expensive! Finding the whole TTC thing pretty frustrating after 6 months :(