condom slipped off

I was hooking up with this guy last night and the condom had slipped off inside of me, i dont know if he finished inside the condom or not. I am 16 years old and am not on any form of birth control, do you guys think i could be pregnant?? I was going to buy a plan b on tuesday but wanted to get some other opinions before i do, especially because plan b is pretty pricey. My period is due in 7 days and i have not been more stressed out in my life, I do not want a baby at 16!! especially since the guy was just a hookup and not a serious relationship.