IS MY GIRLFRIEND PREGNANT?? Need answers!?

my girlfriend and i were acting naughty one day and i might've gotten precum on the outside of her vagina. Not sure though, cause it happened so fast. Exactly a week after that, she got her period for 5 days. It's going to be her third week now IF SHE IS PREGNANT. And she's stressing herself thinking she is.. I just need answers if she is or she's not pregnant cause she's going hysterical.