There is no time in your cycle where you can have unprotected sex and be absolutely certain you will not become pregnant. There are just times when the risk is much greater than others. Take a pregnancy screening test and make sure you follow the instructions on how long after the exposure you should wait before taking it.
The web page below has nine links at the bottom. If you understand all of the information on those pages, you will clearly understand how it works and the risks of different behaviors.
Planned Parenthood:
http://www.plannedparenthood.org/info-for-teens/pregnancy/am-pregnant-33831.asp
