Why is this girl acting weird for?

So theres this girl I work with and Im having sex with her best friend anyways, me and this girl used to flirt and joke around and generally have a good time when around each other, however when she found out I was dating and hooking up with her friend she became a big time hater, like telling her that Im a flirt at wrk and that I love to talk to a bunch of girls and be flirtatious. And now when we are at work its just awkward because I know she runs her mouth to the girl that Im hooking up with. I just ignore her and for the most part she ignores me as well, however sometimes ill catch her looking in my direction and shell just stare, for like 3 or 4 seconds till I look at her, and then shell turn away, whats her deal?