[ Singles & Dating ] Open Question : I honestly don't know what to do?

I have a gf and she told me that she is going to participate at a 15th birthday party of her friend and the bad thing is that she is going to have to dance with another guy so I told her that I wasn't okay with her dancing off with some other guy and then she got all bitchy and told me stuff like "i don't care about her feelings" or "your acting as if were going out as a date" or such as "it's not like were gonna be dancing all over each other" so I don't know if I should let her go? Maybe I should? Maybe I'm just being too over protected and insecure? but at the same time I'm not okay with her dancing with some random guy I don't know and we have a far distance relationship going on since she had to move =/ So yeah I don't know. What do you think guys?