New to opks

Sara • River Ann born on August 9, 2017 at 5 lbs 13 ounces and 19.5 inches long ❤️ my world
So, I took an opk today and it has a faint line. Is it possible to have this AFTER you ovulate? Like a few days after? The only reason I ask is my BBT shows a dip and then an increase from a few days ago, but the app is telling me my fertile window is now. Little confused. Thanks!