How are you figuring out when you ovulate?
Any time you have sex, especially unprotected sex, you risk the chance of getting pregnant. Emergency contraceptive, like Norlevo, is effective, but also keep in mind that it is not meant to be used as a replacement for regular birth control. Also, it is not meant to be used more than twice in a whole year, as it is a huge dose of hormones that shocks your system. If you are not trying to get pregnant, why are you having unprotected sex? Since your period is late, take a home pregnancy test. Taking the test with first morning urine is often the best(most concentrated, making it easier for the test to pick up on hcg levels), but is not necessarily 'needed'. Take the home pregnancy test and then go from there. Hope it all goes well.