About 2 years ago someone I work with made a real effort to become friends with me, coming by my office everyday, being very supportive about work issues and inviting me to do things at the weekend. We got on really well and I thought she was lovely and I had made a new close friend. But after about a year she started distancing herself from me and refusing invites to do social things. I tried to probe about whether she had things going on I didn't know about or if I had done something wrong but didn't get a reason.
Then the distancing turned into outright meanness. She was very unsupportive about a stressful personal situation I had, everytime we met she pointed out my negative qualities and never said anything nice and generally turned into a bit of a bitch. Again, I tried to get to the bottom of it but got told that I was misremembering her as a nice person and actually she wasn't like that at all.
I became very wary of her and generally tried to keep out of her way, thinking I must have horribly misread the situation. But over the past couple of months I have notice that she is now actively seeking friendship with another colleague in exactly the same way she did with me. Part of me is thinking that I can't be a very good friend if I managed to turn this nice person into a bitch, yet I can maintain friendships with lots of other people, and another part of me is just confused by her.