Why did she do a 180 on me?

Anonymous
OK so I became friends with this girl through a buddy of mine she recently broke up with her boyfriend. I invited a bunch of people out to a local club including her and we had a blast and we became really close and even ended up kissing by the end of the night. For the past week we have been talking and hanging out as normally and she said that she was fine with us getting to know each other and taking things slow and even agreed to dinner just the 2 of us and she was excited about. now a whole bunch of us went to the movies the other night and her ex saw me comment on her facebook profile basically saying hi and he started texting her and her friends during the movie asking who I was. the next day she told me that this was moving too fast and she wanted to just be friends. I'm very confused and I never intended to go for this girl but I grew to like her and want to get to know her slowly I don't like being pushy when getting to know someone but yet I still got this outcome. what would you say the problem and/or reason is.
Why did she do a 180 on me?
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