Why does this girl suddenly act totally uninterested?

So I met a girl for the first time at uni last week, and we were talking for almost 2 hours. Turned out we shared some interests like playing guitar or being vegetarian. I even asked her is she would want to play guitar together, to which she replied "anytime!"
Overall, I felt that the conversation went really well.
Another day later I tried to sit next to her and start a conversation. However she was only talking to her friend, and I couldn't make my way into it. I tried a few times, but gave up on it eventually. They never tried to involve me in any way.
This week I sat next to her in another lecture and tried to start conversing, but she only gave me one word replies and seemed to put no effort into it. I mentioned how I had 4 hours of free time inbetween lectures (We both commute to the university, and it's not really worth going home for that time. So we both have a lot of time to kill), but she just said "how annyoing". When I asked what she's about to do, she said that she will meet up with a friend.
This morning I saw her at the train station, but she didn't even greet me and sat herself far away from me.
Now I am wondering what on earth happened and how I should behave.
I feel like my perception must have been wrong. I assumed we got along well, and now she seems to be completly uninterested. What happened, or what did I do wrong I wonder.
Should I straight out confront her, or should I got silent aswell and not chase after her?
Because casually starting a conversation doesen't seem to work at all.
Why would someone behave this way?
Why does this girl suddenly act totally uninterested?
21 Opinion