my experience in league

so why is it today i played 1 game had a great score and then i lose 6 games for 6 loss why is it so hard to win a match in 5 v 5 not to mention the fact that yi is so over-powered its almost annoying as hell and so freaking irritating now i know this may seem like to say but i have played this game for several month and never have i lost so many mAtchs in one day if anyone has experienced the same thing as me please respond below