How do i know my bff is talking about me beind my back?

Well it started when I found out from some really good friends of mine that the person i thought to be my bestest freind ever was asking all my good friends "ew why are you brookes friend?" It made me close to tears to know that that person hated me. But this isnt about her. Its nabout my bff Courtney. We confronted the girl the next day and her face got flushed and she called me a liar. WHAT DID I DO!?!? Then she cheated on my bff for he guy and she used me to get to my bff! and she still thinks were friends. ugh. Then her and courtney started getting all tee hee hee your soooo funny!!! at school. and when i talk to my bff at home she goes ugh i hate ashlee!! Should I tell my BFF bye or just ignore 'em both??? I mean I dont wanna say bye to my bff thats been my friend since 4rth grade!!! even though we have been going through some tension lately... I just need to know what to do!!!