You can NEVER be sure if you have unprotected sex. During the intercourse there is always a bit of semen leaking out, from the very beginning of sexual intercourse- men usually do not know it, due to that, so many pregnancies occur as surprise. This quantity of semen is usually not enough to make pregnant every woman, but some women are very fertile, especially when they are young, and for them is even that quantity of semen that you are excreting during the intercourse and before orgasm, more then enough for pregnancy.
Get the morning after pill immediately and in future ALWAYS use protection. Protection is only way that you can be 100% sure that nothing will happen .