This girl just turned on me i think i need advice PLZZ HELP?

So yesterday at my school dance i asked this girl who ims me 24/7 2 dance and she does and today she posts something about her boyfriend named garrett on her facebook and im just thinking WTF!!!! first of all she is always talking 2 me, and nobody named garrett goes 2 our school. If u think she lying or y she might have done this please answer help is really appreciated i've liked this girl 4 a year now and yeah thank you for all your help.