I had protected sex a week before my ovulation but my period is two days late. Why is that?

Do a pregnancy test. Some menstrual irregularity is common and not usually a reason for concern. Almost all women start a period late or miss a period once in a while. If you had protected sex (used a condom), and you were not using any other form of birth control then pregnancy is a real possibility. Condoms are the best protection for STDs, but only about 60% effective in preventing pregnancy.