if gf period was 3 days after your unprotected sex then she is not pregnant ASSUMING when she said 3 days later "i am having my perio" that it was expected and on time WHICH MEANS her next period is due in a coupld weeks from now so any pain she feels is probably not from being pregnant. the reason is there r only a few days when she normally can get pregnant and on a 28 day cycle the most likely will be day 14 from the BEGINNING of her last period. BUT she would need to say what is her REGUALR period cycle; it could be 28 days or 29, 30, 31, 32, 33, etc.
birth control would be the most advisable
if she has her period in a couple weeks u will know for sure she is not pregnant
at this time u could go buy a cheap pregnancy test at a drug store and use it now
i hope that helps