I had sex 8 days before my last period. I had a full period, so I thought, no problem? Well since then, I have NOT had sex, but my period is 4 days late. Which is VERY odd because mine are always on time. Help please!

It's possible that you're pregnant. Some women experience bleeding in pregnancy, especially early pregnancy.

Try a pregnancy test. If it's negative, wait until your next period is due. If you miss that one as well then test again (using first morning urine) or see a doctor for a more accurate blood test.