Best friend keeps lying to me?

OK well Ive been friends with my befriend since preschool. lately shes been lying to me. about important things too. I'm not sure what to do. Ive caught her a couple times and she keeps doing it! like we had a plan to ask these guys to Sadie Hawkins and she said she had pep band but there really was no pep band that night! this is really starting to bug me and i dont know what i should do! plz help!