I NEED BESTFRIEND HELP DESPERSTELY !?

ok, so i have a best friend sam, and we're like closer than close. we know everything about eachother and could literaly sit and talk for hours and not get bored. she is like my sister no joke, but she has lately been getting really weird, like she's been getting mad over absolutely nothing and let me say she is so pretty, but hse actualy sitts there and says she's ugly, when she's like prettier than me adn most guys i've liked like her. and she keeps like acting weird like she's obviously trying to get attention. well this year i changed alot, puberty really kicked in and i grew into my face and am more confident with myself, sam has ALWAYS been that way, but she didnt change over the summer like at all really, but i was like surprisingly drastic. but yeah i kinda think she's a little jelous of me cause im getting more attention than last year whne i was her ugly best friend. but yeah sam's pising me off big time; i showed her a picture of this guy i really like at my church and she was like wow didnt expect that ( like i couldnt get someone as good looking as him. ) and i was like what ? and she was like nothing and i was like do you think he;s hot ? and she was like making this face like she thought so and then she got all mad randmly and was like im not gong to homecoming and i was like wtf ? why ! and she wouldnt tell me and then she started getting all mad and she was like i dont like nick ( her boyfriend who used to be in love with me but i didnt like him, i hoiooked them up ) and i was like what are you talking about ! ? and im sitting there like all confusded and then i was like im taking taylor (one of my other bff's that goes to a different school ) to homecomign :) we'll all get to hang and she was like oh im not gong its not gonna be fun i wont have anyfun and i was like what are you talkingabout ? you know taylor, you like taylor ! what is your issue ! ? ! ? and i was like whatever and then we went to differnt cles then at lunch she bought her ticket for homecoming and she was like oh well i guess im going and i was like yah. guess so. but she's so weird like having these random mood swings and like getting mad at me for no reason one second and then the next second she's back to normal. it's like really upseting me cause she's not the same i dont feel like i can tell her anything without her turning the convo onto her or she gets bored or something. and she's very judgemental like if i get an a on something and she gets a b she gets mad cause she did worse than me like she's like omg i cant believe i did worse than YOU ! and im like thanks. its horrible cause she didnt used to be like this, and i dont know what to do. should i say something to her or should i just let it go ? & what should i say if i say anything ? Please help im desprate ! :(