Anytime you have unprotected sex, just assume you're going to conceive.
Take a pregnancy test.
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:
If you're sexually active, you need to understand everything on this page:
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.