If you took something like Plan B within 4 hours I doubt that you're pregnant. The purpose of that pill is to block conception. There's no point in freaking out now. Just wait a few days and take a pregnancy test to confirm, but you're probably fine.
Anytime you have unprotected sex, just assume you're going to conceive.
Spotting after sex is normal. Here are some pics of the cervix so you can understand what's happening in there:
http://www.beautifulcervix.com/cervix-photo-galleries/photos-of-cervix/pre-and-post-coitus/
Plus, you might want to learn enough about your body to understand when you ovulate so you can avoid sex before and during that time. This site and the book (available at most libraries) can help you with that:
http://tcoyf.org/
If you're sexually active, you need to understand everything on this page:
http://www.plannedparenthood.org/info-for-teens/pregnancy/am-pregnant-33831.asp
Arm yourself with knowledge and you can do a lot to avoid going through this panic in the future. Always use at least two forms of protection even if it's just a condom and a spermicide that works with the condom material. Don't use nothing, and don't rely only on condoms.
