is she using me ? girl in class college

ii know its long but comon i need help so badly T.T desperated

i talk to her last weeks class in college for the first time
shes really nice to me in class she talk to me first in class most of the times
this week i arrive to class sit next to her and she look at me like she want me to talk to her and i say hey she says hey too when the class was over i looked for her and ask her to add me facebook she says ok
and out of the sudden she walks away without saying good bye or even looking.
idk why she talks to me so much in class and when class is over she acts like whatever
the guys in class are all fat and ugly and im the one that dress better and gentleman with all girls in class.
yesterday i say hi to her on facebook she doesnt reply i say u there after 30 minutes and she says hi and log off

does she likes another guy in class and just talk to me in class to make the other guy jelaous? cause other dudes in class are shy u know im the one that have the courage to talk to her.or does she like someone else out of the class and is confuse

girls so complicated T.T