Why is she so cold and distant all of a sudden?

Okay, so I have this interesting "relationship" with a girl. She and I met online a year and a half ago (We're both from the US, but right now I'm studying abroad for a couple more years, but we plan on meeting). When I met her, I initially had romantic interest in her; I didn't act on it at first, until several months later, she told me that she had a crush on me. This prompted me to pursue her. The first time I tried, she told me she was still in love with the same person she was in love with until she was 14 (she's 23 now and that relationship ended about 4 years ago). So I let it go for a while. A few months later, she informed me that she has been harboring feelings for me and we were happy happy for a little while. However, shortly after, she said she felt indifferent about us because I lived too far away... and I made a blunder and dropped the L bomb on her. This terrified her, causing a strain in our friendship and she has tried "running away" from me twice. Now, I was accepting about all of these and let her do her thing. Surprise, surprise, she always returned several days later, stating that she wants to remain my friend. After surviving the fatality that my love confession caused, we became closer... closer than we've ever been. And for several months, I've noticed that she seemed to be her happiest (she has been depressed for years). We expressed interest in each other a lot more intensely this time (she has even expressed jealousy over the girls that I had been involved with) and we were just really happy together in our own worlds. However, last week, she hasn't been around and on April 27, 2011, I felt something for her... and I knew that I have fallen in love with her. She was absent for 6 days and I sent her a message on Facebook telling her that I missed her and she came on the next day. I've talked to her, but I noticed a slight coldness from her. She seemed to make an effort to be warm and accomodating, but she left abruptly, promising me that we would talk again soon. She kept her promise and I talked to her today, but today was markedly cold and left even more abruptly than she did the last time we talked. So now I am confused. We were just so happy together and then suddenly she pulls this? I asked her if she was okay and she said she was, that she just wanted a change in scenery... etc, but I think there's something more. Help anyone!?