<human>: Can you provide a brief rundown of the text in within 3 sentences?

Throwaway because she knows my Reddit account:

I was really good friends with a really great girl for a few years, and at some point plucked up the courage to ask her out. We went out for about a year, and though we always had a great time together, we both had a nagging feeling that we weren't meant to be together long-term, and decided to break up. We were both very upset, but agreed it was for the best, and we were able to become close friends again, after the appropriate period of separation and so on.

This all happened a couple of years ago, and since then, we've been out with other people. We still talk a lot, although not to the detriment of our other relationships, and I still really like spending time with her. As far as I know, she really does too - at least, she always seems really happy when she's around me.

Recently though (in the last couple of weeks), she's stopped answering my messages as enthusiastically, and yesterday announced by Facebook status that she had planned a holiday with two other guys to Argentina for ten days (we live in London) and was leaving tomorrow. I feel a bit hurt, not because she's gone off without me, but because she didn't tell me, and I feel like usually she would have done.

I can see that being "best friends" with an ex, or even someone of the opposite sex, is a bit of a minefield and pretty risky, but I've long since stopped thinking of her as anything other than one of my closest friends. What do you think might be up with her? Nothing has happened to spark this change, as far as I know, although if it has, she may just not have told me, since she's been distant for the last couple of weeks.
<bot>: Have been best friends with an ex-girlfriend for a long time. Suddenly she starts acting distant. What's up?