Ex-girlfriend tried to ignore me as we walked by each other yet answered, why?

My ex-girlfriend broke up with me two months ago. On our monthly milestone, I got jealous about a facebook post where she appeared to be possibly planning a trip to see a guy for next year. The guy said that he had two tickets to the INDY 500, and that if he would've know, she could've flown up and gone with him. She responded, "Bummer! Maybe next year! ;-)" The wink was what got me, that and we had been together that day. I felt like she would've rather gone to the race with him. I admit that I handled the situation the wrong way, I sent her three e-mails telling her that I felt diminished and suggested she meet up with this guy when we would be in Chicago together the next week. One day she wants to spend the rest of her life with me, the next she cancels a trip we were taking to Chicago the next week and breaks up. After trying to talk to her for a few days I backed off for a month. I sent an e-mail apologizing and said that if she wanted to talk to let me know. A week later, I told her that I wasn't going to expend anymore emotional energy on the situation. I also asked for a momento from a special time in my life back that I had given her. I told her that she could leave it at my cube when she was going to be in the office a month later and I wouldn't be there. That got a response, and not a nice one. She told me that I could cease with the "lectures" and the "letters" (six maybe seven e-mails and one letter over two months, not all at the same time) because at that point, it was beyond necessary. So, a month passes and she leaves the item, and I don't contact her. That was a week ago. She works from home so we don't see each other, but today she was in and as I rounded a corner we were headed straight for each other. She was talking to another girl, and usually when your walking and talking you look where you're going...then look at the person...then look ahead...you know. She kept looking at the girl she was talking to, then looking down, etc. A very, very deliberate attempt to ignore me...I kept looking at her to make eye contact and say hi, but she wouldn't look at me. As we passed, I said, "Hey ####, How are you?" She turned her head toward me without making eye contact and said, "FINE!" in a quite hostile tone. I was already past when I replied, "Good." I've been focusing on myself for the last month and a half, and I haven't made any contact with her, just working on me and moving on. It's been hard because I really wanted to marry her, I thought she was the "one". So, if she wanted the break up and it's been two months, why would she act like this? I mean, why is it so hard for her to just say "hi" and be cordial? Why would she answer if she was trying so hard to ignore me?

Really, I'm curious about her behavior. It's been almost 2.5 months since the break up, I told her almost 1.5 months ago that I got it and best wishes. No contact from me since. She wanted me gone, I've been gone. We'd agreed to be cordial at work, so...