it all depends on when you ovulate. every woman is different. if you don't get your menstrual cycle in the next couple of days I would take a pregnancy test. or you could just be a couple days late stress can do that. but I would wait about 3 days and if you still haven't got your period. take a home pregnancy test
If you have unprotected sex, there is always a chance of becoming pregnant. No matter when you did it.
It can happen if you ovulated later than usual. Your period is probably just late. How many days are normally between your cycles?