OPK, I NEED HELP!!!

I’ve always been the one who can NEVER remember when I started or ended my period. After using the Marina (2) for 10 years, we’ve decided to expand the family. I started using this app to keep tract of my periods after taking the IUD out. We’ve been trying for the past 3 months and nothing. I decided to start using opk but idk if im doing something wrong. Example; I took a test one Saturday (it was negative) I took a test Sunday (it was positive) I took another test Monday (today, and it was negative)!!!! I thought you’re supposed to have like a 24-48hr window to try before ovulating. Does this mean I only have one shot? Please HELP im so confused.