Which day is O?

A few days ago, my FF had CD19 as O day. Today, it moved to CD17. I’ve heard of fallback rise patterns but I don’t know if that’s what this is and I ovulated CD17 or if I ovulated CD19. I got a peak with CB advanced on CD17. I got positive strips CD17 and 18, 18 was a bit darker that 17. Thanks for your help!

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