TTC and confused

My husband and I are freshly TTC. I was on the pill for 5 years and stopped taking it in June. I finished out that pack and had a regular period July 17-20 with no withdrawal bleeding prior to that. According to the app I am in my fertile window and am supposed to ovulate on Thursday. I have just started using digital OPK and have yet to notice any sort of line at all (just a very very faint line). I’m not super worried as I am still somewhat fresh off of the pill, but have other experienced this? Was the app way off with predicting your ovulation? Also, how long after stopping the pill did you become pregnant? Thanks!!