Why would she ignore me now if she was never interested all along?

This girl is obviously upset with something that happened between you two and maybe you should talk to her about it! text her if she wont talk to you in public, tell her how you think she's been treating you and tell her you dont know what you did and make sure she knows that you know shes not interested and youre okay with that..(if you are) and maybe she will appreciate that and you two can become friends and on speaking terms... but if she likes this other guy and you know this for sure, then back off a little bit... you dont want her to get the wrong idea. good luck.(: