I had sex with my ex boyfriend of four years last month (March), this month we did have unprotected sex. Now my period has not come on? I thought maybe it could be pregnancy but I have not been able to get pregnant for three years now. What else could it be?

When was your period expected? Just because you haven't been able to get pregnant for three years, doesn't mean that you aren't right now. Have you taken a pregnancy test with first morning urine?