Delayed ovulation

I'm on cd 17 today and no lh surge! I'm so confused!! I'm usually very very regular with ovulating at least cd 15, never later. I've been taking opks but they show such faint lines, not showing I'm anywhere near ovulating! I really don't know whats happening! Does anyone know what might delay ovulation?! I started taking prenatals this cycle, could that have done it?