It is possible you either ARE or WERE pregnant. Two Things: 1. "Chemical Pregnancy": It is estimated that around 80% of first time pregnancies 'abort' automatically by itself (miscarriage if you will) in the first month. It is estimated because most women do not have any 'serious' signs of miscarriage. Many will have an abnormally more serious 'flow' (bleed more), and others have found out because they tested positive with a home pregnancy test but then went to doctor and doctor couldn't confirm the pregnancy. Its called a chemical pregnancy because the cases we do know of are of the last kind, a chemical signature of human chorionic gonadotropin (hCG) at a level that can only be had through being pregnant. 2. You are pregnant: It is possible that you can have your first period after conception without risking the health of the fetus. Further, about 10% of all women experiencing some type of light bleeding during pregnancy. This bleeding, though, is not the same as getting your period. During early pregnancy, the cervix changes in order to accommodate the baby. The body will provide increased blood flow to the cervix, and sometimes this can result in light bleeding. Lastly, Which came first? Was your expected menstruation before sex or after? If before, then there is no way having sex with him or any man afterward can affect that. If after, then it is possible that sex can effect the time of the month. If sex came after the expected period, then you missed for some other reason. 10 reasons why you can miss a period: http://pregnancy.about.com/od/amipregnant/a/missedperiod.htm Mind pregnancy is only 1 out of ten.