I think her "fuck you" remark was her being superrr defensive, which gives you every right to be mad because if she's being defensive, she obviously knows she did something wrong.
you need to just confront her face to face. If she's out flirting just to get the attention, then maybe you should rethink your relationship with her. attention seeking girls cause more problems down the road.
++vibes. fighting with in a relationship blows.