Interesting responses so far, I'll weigh in here with my opinions.
The first and most common IMO is she is pulling the **** test, I wouldn't even say it's intentional. She's not trying to make you jealous per say, but she's trying to see if you're the jealous/controlling type. I personally don't have a problem with my chick flirting with other guys as long as there is only one c*ck she's riding at night, I still talk to/get numbers from other chicks I find interesting even if she';s out with me, I don't think it's unreasonable to expect her to talk to other guys either. For example I am going out to this chicks birthday tonight, we're not dating but I'm still expecting to be the one taking her home when all is said and done, that being said I fully expect that other guys will be buying her shots and making moves at her, likewise I will be with the other girls in the group. There was a very good post I read awhile ago about creating competition anxiety between women, there's a reason women seem to be all over you when you're in a relationship when once you've broken up they don't seem to be as plentiful.
Second option, she's testing the waters because she's ready to swing branches, simple solution here, NEXT. Some of the comments here remind me of myself a few months ago, Long story short, Why worry about being jealous or if you have to next her when you can just go out for a good time?