Needing some advice about figuring out when I ovulate. Last cycle, I used opks for the first and it was quite straightforward. The bottom line for darker in cd11, cd12, and my cd13 it was darker than the top line. Luckily I tested a couple of times as it got darker in the evening in cd13. This cycle, the bottom line isn't even appearing. It is cd15 and I have been testing a couple of times as day since cd8, and nothing. Could my ovulation day change drastically each cycle? Thank you!