Why is my best friend behaving like this?

Me and best friends are together from past two years,though she is my junior at school.we are the best of friends.my old friends always warned me about her,but I never took it seriously.lately a new guy joined my class who is her senior.he is handsome and I had a crush on his best friend but now I got over him…ever since the guy joined the school I got to know he speaks about me in a good way.he tells people she is a good looking girl and she can get a better guy and all.I used to share every secret with my friend(who is a girl).she always used to say this guy is better looking but im not interested in him.both of us used to pull his leg,all though we never spoke to him ,we used to laugh at his face.he never mind that…later me and him became good friends..he approached me first,he used to indirectly flirt with me,he used to check my a social site profile..I used to tel my friend every single thing we used to chat..all though I never had feelings for him..but I never knew what was on his mind.I told him that me and my friend enjoy pulling your leg..he said he doesnt mind..when I pull his leg he enjoys but but when my frnd does he completely ignores her,infact he runs away seeing her,when my friend tried to talk to him he turned his face away.my friend gets irriated by that..she keeps defending herself I dont like him..when I do something he gets mad but when you do it he doesnt mind,why???.im more popular and prettier than my friend and I some how started to feel she wasnt liking me talking to this guy..but this guy is a friend of mine..he made me forget my ex and when im with him I forget my worries..he has a girlfriend too..lately my friend told me to ask him why is he scared of her.he gt mad he said yes im scared you please ask your friend to spare me..do you think she luks scary??he asked me that…later he stopped runing away from her and stared lukin at her more confidently..and my friend got excited about it ,she says now he is behaving like a man…when he told me to tell her all that..she started fighting with me…she got so aggressive I couldnt believe she would behave like that.I was like what did I do?I seriously dont know the reason for her behaviour.I was like if you dont like it I wont talk to him I dont want to lose you..she became sarcastic no I dont want to disappoint you talk to him any way I don’t like him..I was like why are you justifying yourself so much?if you dont have anything for him…the guy always changes the topic whenever I talk about my friend.and also gave me his number..he said you can contact me through this..I didnt tell my friend about it because I know she wouldnt like it.plzzz guys why is she behaving like this?was she just using me because im her senior and popular and influential?why is she fighting with me for a guy whom we hardly know?what does that guy think about her and me?should I trust this girl or not?