Late period

So my period is late by 3 days. When should I get worried? I have protected sex (with condom) but am not on birth control. I know there's still a chance of me getting pregnant even with using the condom. I'm trying not to freak out, but when should I be getting worried? I'm also taking some college courses so I figured I could be under some stress but I don't feel overwhelmed or anything but it still may affect my period.