Hello, I'd like to help you with your question.
If you are witnessing her behavior yet she is denying it, then she is trying to make you feel like the wrong one, most likely because she doesn't want to face what she is doing. When someone blames their partner for something they do to hurt them, it can border on emotional abuse. It is basically a way to ignore your feelings in preference for her own, which hurts you and gives your partner permission (in her own mind) to continue her behavior without interference.
Most people who feel the need to flirt do so because they feel insecure. They want the attention and they also like to potentially create conflict with their own partner and the partner of the person they are flirting with. This invites dangerous behavior so they do it in such a way that it is subtle and hard to accuse them. This also adds
a sense of superiority because the person feels they are "tricking" everyone else.
You can try confronting your partner again by saying what you saw. If she still denies it, then say "I'm sorry you are not willing to face your own behavior and deal with my feelings about it" or something similar. This lets her know that you are not going to accept being told you are wrong and that you intend on sticking with what you witnessed.
You also need to decide what you are willing to do about her behavior since she seems unwilling to accept responsibility. One option is to leave when you witness her behavior. Another is to refuse to go out with her until she addresses what she is doing. You can also consider seeing a therapist to talk about how you feel and see if she is willing to go with you. A therapist can help you sort out what you are seeing and what might be caused by insecurity.
No matter what you choose to do, believing in yourself is the most important step. There is something going on here and regardless of what it is, you deserve to have it addressed so you can feel better. Hopefully, your partner will support you and not continue to dismiss your feelings.
I hope this has helped you,