I experienced a similar situation with my then-fiance with her coworker during our housewarming party, and I could sense something was off. What you're going through is valid, and you are not overreacting; it does feel different for a reason. Don’t let anyone tell you otherwise. She claims it’s harmless, but you need to express that it's not—it's hurtful. Flirting often leads to complications, and she shouldn't be engaging in it.You didn't mention how long you’ve been together, but from my own experience, I suggest cutting your losses and finding someone who truly values and respects you, someone who wants to be with you and not flirt with others. Flirting inevitably results in someone getting hurt, and there's no need for that. Respect yourself. This time, you caught her in the act. Just think about how many times this might have happened before. If she views flirting as harmless, how would she interpret something like cuddling or long hugs? Best of luck to you.