So does this mean I still have a chance

I know some women get a positive pregnancy test before their period or a few weeks later I know it depends but if it's been 3 weeks after sex shouldn't it be accurate by now? I have a irregular periods and got a bfn I did use opks for the first time and I had sex around April 14 which is the day I got a positive opk and today the test is negative if no period do I just repeat and test again?