I usually get my first positive opk on cd 12 and it lasts for 3 days. I am sure I ovulate on cd 13 since my cycles are 26 days and AF is always on time. This cycle I didn't even test until cd 10 and all of a sudden it was positive and has been negative everyday since. I'm hoping we have/are bding enough as we did cd8,11,12 and will tonight. Has anyone else had ovulation be the same time for months and then it suddenly changed?