Late...

My period is late by 3 days.. Yesterday I had sex for the first time since my last period, and now I'm paranoid and can't think straight.. I've recently (in the last 2 months) got my implanon impant taken out which has made things a little irregular.. Along whether that I've been under a lot of stress and pressure with exams.. I know everyone is different but how late is late to be worried about??..