Am really late on this thread, but recently read that your body can work itself up to ovulate and start hormone release (pos opk) then something happens and it changes its mind (often eg stress or illness apparently, so your body knows when to hold off on conceiving!), then a few days or weeks later it will have another go so you will see another spike on the opk. If you get more than one spike then you can only have ov'ed ont he last one as it was your body getting ready.