Girls, Why is she so cold after rejecting me even after giving her space for friendship to resume? Help me decode what I did wrong?

I was on this course with this girl, to whom I was a good friend to. She was passionate sparkling and bit dizzy which was charming Over time I developed feelings for her as I saw her in a new light, but was pretty sure despite some mixed signals she only saw me as a friend - "the coolest friend" in her exact words. Eventually she dated my flatmate which lasted about a month which was a pretty rough time for me which I kept hidden as I had no right to interfere it was her choice after all. Eventually they broke up after about a month. After a while I had to get her out of my system and move on. I found out from her circle of friends that she had a list of 10 men she fancied. So one night at a bar I asked her straight "this list of ten men... I'm not on it am I?" She said no. I must have looked really crestfallen because she apologised and said "I'm sorry". I said it was Ok no worries. Chatted a bit more for a bit changing the subject to avoid the awkwardness, licked my wounds and left the bar. I Determined that night that I would move on but keep it civil with her after all she was a friend.

But the next day when I met her and her circle of friends on Campus and said Hi in passing they all ignored me. Eventually her friends got friendly again, but she stayed coldly civil, with no warmth, Never actively engaging. I got a girlfriend moved on and she found a new boyfriend too and I hoped that this would change her attitude, but she still stayed cold. Not spiteful or resentful. Just weirdly guarded. Most of the times girls get annoyed that men don't stay friends after rejection I gave her space but the friendship never resumed. Until now. Uni has finished now and we have moved on with our lives. But now out of the blue she's starting to contact me again after making no effort. Nothing can possibly happen now but I'm annoyed I might be getting mixed signals again. I can't decipher what I have did wrong. Her reaction after my sideways confession is especially puzzling.

Girls, Why is she so cold after rejecting me even after giving her space for friendship to resume? Help me decode what I did wrong?
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