Why is she being like this, help?

We have been best friends for a long time now, we haven’t always been the best of friends as she can be really horrible. But we both like this guy who goes to our school, we hardly ever talk to him at school but we always talk to him on a social site. Whenever he’s online she either wont talk to me or she gets really mean and shes always telling him bad stuff about me, which isn’t true, she also tries to talk to him as much as she can so he doesnt have chance to reply to me and she tries to get me really annoyed so I just go offline so he cant talk to me. And at school, if she wants to say hi to him she always leaves me and goes off with him but I atleast take her with me when im going to talk to him, even thought she likes to get all the attention and doesnt really want me to talk to him and whenever he talks to me she always has to come in and stop our conversation. It’s really annoying because I like him a lot, and im pretty sure she does aswell, but I dont understand why shes acting like this. :/