confused with my opk results. please help!

I tested positive opk on cd 15 evening, cd 16 morning. Cd16 evening however was negative. 
But cd17 morning is now positive again. 
I'm on a an extremely regular 28fay cycle and this is my first month ttc. No pcos or anything abnormal detected from my recent blood work and ultrasound as a routine check up. 
Have I already ovulated? Not yet? Ovulating twice? Internet has got me confused.