What would make her do something like this?

so there is this girl that i like that i have cl with, and one day she starts sitting next to me and flirting with me she also puts her legs on mine and touches me when she reaches accross me. we went on a field trip and she came up to me and said she wanted to sit next to me on the bus, we started texing but not everyday only every now and then(i always have to text first), so im pretty sure she likes me and i waited untill after spring break i decided i was going to ask her out, so on friday i asked if she wanted to go to a movie with me and she said she didnt know if she could because she had to go to some college thing and i told her to call me when she got back. then on saturday i sent her a text to see if she what she was doing and she never text me back or called me. i dont know why she would she would just stop talking to me, im not sure what i sould do about this. i saw her today and i didnt bring it up, she was acting weird the first half of cl but then she still put her leg on mine what should i do