Opk positive to negative over night?

First month ttc and I started taking opk last Monday due to app saying I should be ovulating around the 19th, every test was negative (I think!) instructions said negative if only one line or a very faint second line, positive of test line is as nearly as dark/darker than control line. Tests have all been negative except for one yesterday afternoon which I took after getting bad cramps, test line was much darker than it has been but today it’s gone back to being light....can it change that quickly? Am I likely to have ovulated yesterday? Thanks!