@gsfak,
hi there! there's no easy answer to this question especially since you guys have only been dating for a few months.
i think you should get to know her a bit more before you can judge. normally, if a woman cheats this early on, it could be a bad sign. but how was her dating life before she met you? was she careful with who she sleeps with? if so, maybe this time, some emotional baggage that she carries from the past triggered her and she made a mistake by cheating on you. either way, you should talk with her thoroughly and try to figure out together why she did what she did. it's good for both of you to discuss that openly.