Why does my friend talk bad about me to other people, but when i confront her she denies it?

I have this friend i have known for 32 years now. She and i have always been good friends that i know of, but just in the last year or so she has been telling lies behind my back to other people and most of all my boyfriend. Me and my boyfriend got in a fight 2 weeks ago and he ended up calling her to spew about me then she started saying vicious lies, that were completely unfounded. Why would she do this and what in your opinion should i do? Really lost and upset please help.