this thread has helped me too.
This happened to me with a girl I thought was a really close friend. We shared a flat and she joined my circle, had lots of mutual friends and had a great laugh together. Then one day - completely out of the blue, she had been lovely in the monring - I walked into the kitchen and chatting away to her and she walked out without a word.
I am usually a pretty robust person, but was really shocked and hurt. I thought I must have done something (totally inadvertently) wrong. But she absolutely refused to look at me or tell me what I had "done". The only thing she ever said to me again was "I was brought up to be polite and i was taught that if you can't say anything nice, don't say anything at all, so I can't speak to you" .
It really cut me up at the time. i was so hurt, especially as home was not a lovely sanctuary at the end of the day. I'd get in after a sh*tty day and there she was, radiating waves of hatred towards me and staying absolutely silent, clutching the remote control. So silent that she wouldn't even pass on phone messages to me, one with a job offer, which I lost out on . I lived in my room.
Friends said "Don't take it personally" but it was personal. She was her usual fun and chatty self with all our friends who noticed there was something up, which is what she intended. They asked me what was wrong and I said truthfully, that I didn't know, had triedn to talk to her etc. They asked her what was the matter and she'd sigh, look tearful and say that she "couldn't say, don't want to stir or for you to think badly of GryffinGirl". She would refuse to come out if I was there and and everyone thought I had done something terribly hurtful to her and it was all my fault. Eventually they stopped inviting me. It was all so wired!
I realised after soul searching, and thinking that surely I must have done something to deserve being treated like this, no matter how unknowingly which I could apologise for and make amends, that I had done nothing at all. She just decided she wanted my friends all to herself and mounted a campaign to "oust" me. It destroyed my confiednce for a long time and even made me question my personality because it must be "me" .
There was no escaping her, but eventually i managed to move out and to this day if I am very stressed, I will dream about her .