As far as i know it can change that quickly, which is why they suggest you test twice in a day... main reason being that the opk only tells you it has detected a lh surge, but is (sadly

) unable to tell you wheter you are at the beginning, middle or the end of it.
Have you tried peeonastick.com yet?
if you've gotten some bd in there, one little spermie ia all it takes, remember, and those suckers can live a long time (i got pg from bd on a wed and got + on thursday, which usually (!) means you are about to ov within 12-36 hours..
Good luck, hun, it can still happen !!!!
Omi xxx