If you're not using birth control pills, your periods can always become out of rhythm for many reasons. You may miss one altogether or be a few days late every now and then. I know that before I started taking the pill I could never calculate when my period was gonna come. Perhaps when you start having sex there are some hormonal changes occuring in the body which may have caused this. Irregular periods are not unusual though, unless you're on the pill in which case they should be regular as clockwork.