This girl is driving me crazy!?

okay, there is this girl that is driving me crazy, i like her a lot but i cant determined whether she likes me or not. She is very sweet with me when we are alone, smiles a lot, talks very sweetly to me, but at the moment someone that she knows or that we both know gets near, she starts treating me like crap, she immediately tries to ridiculize me in front of that person. She doest that to others too, but my friends agreed that she does it a lot more frequently to me, and im getting tired of she doing that, she makes fun of everything i say and incites other people to do so too. Why does she does this? and how can i stop it? im getting tired of that behavior of hers.