fertile window ?!

Is anyone else's fertile window off a bit ? It definitely does say I'm in my fertile window when I ovulate but my suspected ovulation date is way off from when I actually do . I ovulate today and it's only day 2 in my fertile window !!! I know I did today due to my CM (it looks like a big clump of snot come out) and that happens the day I ovulate every month religiously . And I get extremely horny on my ovulation day .. and all I've thought about today is sex . Is anyone else's ovulation prediction off ?