Why is my friend always LYING?!?1?

firstly, i asked her oh whens your sleepover, and she said on saturday ( she todl me this morning) abd then b ythe end of the day , she said oh yeah im grounded , and i said why, and she said my mum tells me when im ungounded and allowed to go out, and i duno what i did wrong. so then i saidd well then you wont be able to come to my pary, andf she said, oh no cause i will cause its only for a week. also she lies about shes going on holdiay, then she doesnt , and petty things liek that why?!?!