Girls, Why is she acting like this?

I caught her talking to another guy several times. The last time she was asking me to move back in, saying she wanted to marry me and have a family. I caught her in a lie about some guy that just contacted her the night before and she says she just can't do it anymore. I give the keys back, she has him over and basically moved in a week later.

I go no contact after trying to talk to her a few times and she starts asking me if I'll sign papers to close a joint bank account... that has nothing in it. There's no point in closing it. She gets mad that I'm not responding, then contacts me later saying please say something. Two days later she says my name in a text, then one saying she wishes I'd say something. Two days later she blocks me on facebook randomly, then unblocks me the next day. A day or two after that she makes her relationship with the guy public. Then three days later asks me again about the account, saying she just wants to know if I'll sign. This is now 14 days of no contact. So I decide to respond saying that no I wouldn't sign, that it won't affect anything and that it's the last thing I have of "us" so to just leave it alone. Her response is just "okay".

Same day later on, I'm driving to work and I'm stopped at a stoplight. She just happens to be walking to work and is at the light at the exact same time. I had sunglasses on, so she wouldn't have seen me see her, but I see her take a long look out of the corner of her eye at me.

Girls help me out. What is she thinking.
Girls, Why is she acting like this?
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