Depends a lot on your DP and your relationship with him. Some heterosexual people find a partner's flirtation with a same-sex OP much less of a problem than a heterosexual flirtation...
OK at the risk of getting utterly tangled up in pronouns I am going to use the ones relevatn to the OP but please bear in mind that everything I'm going to say applies ABSOLUTELY EQUALLY to women who's male partners have flirted with other men, and same sex partners who have been flirting with heterosexuality. Here are the most important questions, which the OP obviously doesn't have to answer on line but should think about.
Has the OP been wondering about her own orientation for some time, or was this just aspell of pop-culture-induced curiosity?
Does she think her orientation might have changed or be changing?
Is her relationship with her DP a very monogamous one (no flirting, no being alone with a member of the oppostie sex ever - oops that might get a bit complicated now) or a more open one or has it never been discussed?
Does she want to see the other girl again/will she be seeing the other girl again because the other girl is a friend or colleage?
What did/does the other girl think about it all?