Why does this girl STARE at me so much and act like a b*tch?

There was a guy at my work I had a "thing" with ages ago, it didn't work out, we didn't stay friends, I kind of felt he was too disrespectful, flirty with all women, flaky and a bit of a put downer/liar.

Anyway we didn't stay friends or anything.

So though he tried being friendly and chatting to me, he kept trying to flirt with me still, and acting pretty weird tbh. Id just be polite nothing more.

Anyway this girl started at work and she basically hooked up with him (even though she was with her fiance of 6 years she cheated on him) but left him now.

They were so in your face about their relationship, basically the guy tried to make it out to her that I was jealous of her when I couldnt care less! I didnt think much of her because of what she had done to her fiancee.

She had an argument with me one day because the guy had told her what I had said to someone about her (I knew he could hear and would probably sh*t stir, but I didnt care) I just said she was being superficial for dieting so much it makes her ill so she cant do her job properly and everyone else has to work harder. She called me a "pathetic cow! " and then started bringing up the guy! which had nothing to do with what I had said about her.... she then started telling me how she knew what me and him had been through because he told her. WHY THE HELL IS HE TELLING HER STUFF.

Anyway since this apprently they broke up, and she still acts weird towards me at work, she always startes at me.... and doesnt talk or ignores me if she can, I dont care but I mean i think its pretty pathetic im only being polite and asking about work related stuff.

She even tries to be a bit bitchy sometimes like gives me dirty looks, and when I was saying to this other worker today that the thing id been cleaning looked clean she said to this other girl "looks clean but isnt clean" what a *****, !

Why does she still care and act like a b*tch?? and STARE?