I have a few <a href="https://play.google.com/store/apps/details?id=com.glow.android">fertility apps</a> but mainly rely on glow because it helped me conceive once already but this time I'm using opks and I tested negative yesterday on a digital opk and this morning I tested high on a digital opk but wen I charted high on ovulation it said my calendar changed and said I've been ovulating for three days but today was my only positive. Why did is that?