Is she trying to steal my best friend or is it revenge?

Okay Me and my BFF have been friends since 6th grade. Everything was great between us, but then in 9th grade my other best friend introduced me to my boyfriend's ex, Lizzy, and we started talking and hanging out. But each time ma and my bf break up, Lizzy flirts with him trying to get back with him and all that. But me and him always solve our problem and get back together. So now Lizzy hang with my friends all the time and I feel like she's taking my place cuz when I try hanging out with them I feel like an out cast, and she's always around my BFF and she's always with her. And one of my friends told me that Lizzy really is trying to take my BFF from me. Could this be just revenge for her ex bf dumping her and then getting with me?