Possibility of pregnancy?

I, being a fool, had unprotected sex with my girlfriend. We are both 16, we were having sex and i was about ready to finish. When i felt like i was going to "nut" i pulled out and finished on her bed, too my dismay i looked up at her personal area and there was a slight amount of semen on it. Now, not to be too descriptive but i need to know the chances of a pregnancy, so please help out. Right at the opening of her Vagina was some semen, i quickly wiped it out with whatever was next to me, cant remember what it was. Once i cleaned it out (Remember it was on the very entrance of the vagina when you spread the lips), i told her to quickly go check herself. When she came back she said she cleaned herself up and only found very little more. Now what are the chances of pregnancy on this? I tried to get a morning after pill but i could not get one within the 100 hours alotted time that it could still be effective. She tells me not to worry but it is a very hard thing for me to do. I am a huge idiot for having unprotected sex and i know i never will again, but i need to know everything and anything there is to know about this, how long before signs, chance of pregnancy, what i should do about it, etc. Once again she tells me not to worry, but i can't, i'm such an idiot...