This girl i like from school?

Well, ive been interested in this girl for about a couple of months now and i thought that she liked me because she started to look at me and smile a lot when she walked past me in the corridor and when she got off the bus. I did the same, i said hello to her and i smiled back and i was gonna approach her and ask her out but recently when we p each other she just ignores me me and she knows im there she just wont say hello or even greet me :( i really like her i just dont know whats suddenly changed her tune?