why do my friends leave me out?

[Farai]

hi i am 15 and i am sweet,friendly and bubbly person who try her best.i hang out with friendship group with 8 of us in it.my so called friends leave me out constantly they plan things behind my back but sometimes invite me which leaves me confused,but i think it is horrible.recently i am planning a sleepover for all of us on the last day of school but now i kind of regret it because they all planned to go to a german market and they did and i saw photos of them on facebook which left clueless to why i did not get invited.even the girl that gets treated bad in the group got invited.what!!!!.so basically i tired to to catch them out so ui asked one of them what they did that weekend and they said nothing much but then i said looked u guys had fun and she said yeah we did? so then i said my weekend was boring and then she said oh i thought u went shopping could that be the reason they didnt invite me or are they plain horrible bitches help!!!! and should i cancel the sleepover because i feel like a mug