Mid cycle spotting

I chart my bbt regularly and I am sure I ovulate every month between days 14-16. This month however, I used opk in addition to bbt charting. Using Clear Blue digital, I had two days of "High Fertility" indicating estrogen surge but It went back to "Low Fertility" the next day without a peaking LH surge indication. Today is day 15 of my cycle and I have severe headache and spotting. My temperatures have not gone up yet. I am not sure if it is ovulation related bleeding or simply that I am not ovulating and my esteogen dropped before LH surge. Any input is appreciated. Thanks.