Why does my bestfriend act mean and differently to me when were with her friends?

So yesterday my best friend asked me to come to the beach with her friends and so I did. It was fun but my best friend was just bein mean to me. I would say something and she would tell me to shutup or no one cares and no one asked me. And then we were sitting down eating and we were all done and then I said let's go lay down and tan! And she said shutup. But then a couple minutes later her bestfriend said let's go tan! And my bestfriend got up with her bestfriend and layed down and tanned! And then the most popular girl in the whole town came and sat with us and she started asking us what we hate. My bestfriend replied with I hate when zoey is here (that's my name) she was being so mean. Then a couple weeks ago he introduced me to one of her friends who is a boy and we started talking and texting. I was texting him yesterday at the beach and he said he was there so then I toldmy bestfriend that gingie is here and that's the nick name I gave him because he is redheaded my bestfriendflipped out and said I don't even know him and I shouldn't give Jim nick names I don't ever say anything back to her when she say all this stuff. Then we were watching suffers and I pointed out that one of them was cute and my bestfriend said shutup you don't even know him I replied I never said I did and she was like k with attitude. Then her mom wanted her to come home and so did my mom and my bestfriend made a whole thing up how my mom doesn't want me here alone and that since she was my ride home she had to leave to but that wasn't it her mom didn't want her alone and my mom was fine with it. But when were alone it's never like this and we have a great time. I want togangout with them together because it's fun and I dont have any other friends but howcan I make her stop treating me like this.