Just dont worry too much. There are alot of girls just like you worrying about this every hour, so dont stress it.
Just think about a plan, a plan that if u are pregnant, what are u gonna do, and another plan that is if u are not pregnant, what are u gonna do.
If u are, are u gonna get an abortion or not? U have to really think about it, coz it's a life we are talking here, but then again, it's ur choice. I think if u had a choice of having sex at a young age, u have a choice to have the child or not, but dont abuse that freedom.
Becoz u made a mistake once, it's fine, just that u are immature and didnt know better, but if you make the same mistake again, it's dumb! Remember that.
So just go see a doctor they usually have some pretty good advices and ideas. Good luck