My mums invading my space PLEASE help me?

well i havnt been going to school and my mum keeps thinking somethings wrong even though i tell her nothings wrong i had some messages on facebook and stuff about me cutting myself when it was really just an excuse for not going to school my mum keeps acting like she dosnt know my password but every time i borrow her phone shes on my account and just bfore ...(its 3am) she came running down to my room and said youv got some explaining to do your living a double life i dont even know you and i really had no idear what she was talking about but she showed me some messages on facebook that had been sent to this boy whos 20 im 14 btw and it was sexual and stuff and she though i wrote it when it was actually my other friend whos 13 and i told her that but the point is she keeps invading my privacy and its anoyying the crap outta me iv told her to give me my privacy but it hasnt helped please help me and tell me what too do?!