Ok, Im a little embarred to ask this but here it goes...?

I am 16 years old, My girlfriend and I were messing around and she was well riding me and i came, i didnt say anything, so we just kept going and by the time we were done, I got up and my pants were wet. Now 2 weeks later, shes telling me she hasnt started her period yet, i am super worried. First off can sperm go through my boxers, shorts, through her shorts, through her underwear and up her ? and second is there any other reason why a girl would maybe be late on her period? My girlfriend is 15