why keep secrets?

i think my bff has been keeping some HUGE secrets from me. i think that because ill always hear her wispering stuff into my friends ear and then she'll say my name really loud.i have known her from when i had went to a new school for 3rd grade.now im going to 6th grade...,same school,and ever since i went to that school...my very first day...when we became bffs,shes been doing that to me. and everytime i ask her what she said about me she'll say "dont worry about it" or "its none of your bisness"and shes my BFF i have other friends but i hate to admit it but i trust my not so friends more than i trust her.i mean i literally count im my enimy more than my bff and thats kind of really sad and hard on me what should i do? i need help.