What has changed?

Hi,

So about 4 weeks back this girl came up to me, she was pretty agressive in her approach, which I like.
We started texting, and at first i was kind of distant but she grew on me. We flirted back and forth, and went on a couple of dates.
I got a vibe from her she didn't want to take it fast so i waited. At the end of the second date, 3 weeks of texting had past, i kissed her goodnight. Not even making out but had planned to do it the next date, which was 4 days away.
In those 4 days she started to behave differently, less texts, cold distant all that. She cancelled the date because of schoolwork, which I get [we're classmates].
I gave her the distance, started texting less etc. this behaviour continued for the remainder of the week, and she fullout started ignoring me in person ( not even giving me a look or responding to a group question ). Finally I confronted her to ask, 'What has changed??'

She started going on about that she wasn't sure what my intentions were and that i wanted something more. I told her that im not looking for something like that, and honestly, i don't think i gave off that vibe except for caring, even for the trivial things.


To add, the conversation which we had, had a high: 'it's not you, it's me' level to it. from her part. I asked some rather bold questions ['Are we friends or more than friends?', no definitive answer so i asked 'what would you do if I were to kiss you now?', still a vague answer] but basically she said that she didn't wanna commit, and if we were to take it further than a relationship was in the immediate flow of events.
I said my part, we decided to take have a 'free relationship' but we didn't really adress 'us'. Allright, i can live with that BUT to make matters worse, she has started hanging out with another guy { who she has kissed before, but said was a mistake }. The same night we had that party i saw them at her house, and im fairly sure they kissed. Again.
He and I are very different, he is more childish, just blahblahblahblah with no control. I think about what im saying, not to the extent that im dull, just trying to be witty and charming.

Right now she is still as cold and distant, BUT she is warm and cosy to others. Honestly I don't know what to do.
* I am pretty critical of my behaviour, words and actions and I can't find anything I did wrong in those 3 weeks*

Am I wasting my time? Has that one kiss, which was freakin'nothing changed everything SO much that she doesn't have any feelings or interest in me? Am I being used as a rebound or to make the other guy jealous?
Any opinion is appreciated, and thanks for reading.