Why have I missed my periods this month ?

hi i am Jaani, age 23 having a problem on menstrual period . My period was absolutely normal before this month. the normal date of period is 3rd in every month and it run for 5 days. i am in a relationship for 5 years. i have never missed my period time accept this time. At 24th march 2011 we did sex and we used protection as well. i do not know why suddenly my period has stopped !? we are, especially i am really scared right now about pregnancy . for your concern i don have any Sexually Transmitted Diseases, Vaginal Bleeding or any other as far as i know. its been 7 days my period has stopped. What happen to me and how i can get reed of this?? i don know the method which he used but he used the same method what he was using for those years and it did not break for sure. is there any other option without abortion and how can make sure that pregnancy is the highest possibility ? its stopped only for 7 days. should i wait for to start or what could be the best option on this situation. please reply ....please...