I... uh... I'd assume it's a way of her dealing with her shyness in making conversation and a sensitive way keeping your interactions private and intimate even in public.
I've certainly never heard of it being some kind of secret code, your theory about it being a countdown to sex is particularly... well, I'd be a bit scared if I were your shy and sensitive girlfriend.
But, hey, maybe you're right and I just don't know about this notebook deal!