be careful not to let your shock impair your judgement. unless you've already done so, tell her what u feel and give her a chance to respond. she might be so sick of sleeping around that she'll prove to be a wonderfully faithful gf. think about it: she's alot less curious than a girl who has only been with one two other guys. she, unlike a less experienced person, knows what makes a good lover and what makes a bad lover. if she wants you to be her lover, you should feel flattered, not threatened.