I'm sorry to hear about your situation.

I think the most important thing you could do would be to calmly confront her, not by accusing her of betraying you, but by explaining how you are interpreting what you have seen, and how it is making you feel. It is possible that she doesn't believe that what she is doing is flirting, and therefore doesn't understand. She deserves to hear from you why you are upset, specifically, and deserves a chance to explain her side of things. What she does not have the right to do is tell you that you can't feel how you feel. You aren't overreacting. You are reacting. And you have the right to do that.

If she tells you that she didn't consider it flirting, maybe you just need to explain to her that to you, a line was crossed, and that in order to be considerate of your feelings, she needs to understand and respect your boundaries. One of the biggest problems (in my opinion) with flirting on the internet is that it is an advertisement to whoever sees it that "yes, I am available, and yes, it is acceptable to approach me!" That is not the message any attached person should be sending. If she doesn't understand that other people could see it that way, you may want to just flat out tell her.

If she admits that it is flirting, or simply won't acknowledge your side, it may be safe to assume that she is enjoying her behavior and doesn't really care about your feelings. Flirting can make someone feel attractive, desirable, etc. She may also be enjoying that sense of reminding her ex what he is missing. In my opinion, none of those excuses/reasons have anything to do with you. Even if she tells you something like "well, if you paid more attention to me, then I wouldn't be looking for it elsewhere," she is really only stating her own choices, which needs to own up to. As an adult, it is her responsibility to find and select good solutions to problems, that take other people's feelings into account.

I hope that through honest communication, you can work it out. Remember to keep on reiterating how you feel ("I" statements) rather than just accusing her of things, because people tend to shut down when that happens.

I think confronting her is probably the wrong way to go... confronting her implies there will be a confrontation with the accompanying drama, raised voices, hurt feelings, high emotions etc....

But not saying anything isn't the answer either. You're clearly (and understandably) not happy with this. Its the sort of thing that can put strain on relationships even if there's nothing in it at all.

Conversations and communication is really important in any relationship.. so instead of having a confrontation and a big scene, bring it up in normal conversation. Be honest and open about it and see what she says. If there is something more serious then you'll know, but its possible that she wasn't aware that its a problem, and she won't be unless you talk about it.