Command non natural generating on blocks

THE BUG 

WHEN YOU LOG INTO YOUR WORLD... everything is normal but if you place a block it turns into a command block even stairs and trapdoors! your skin goes all glitchey and cubey and mobs just spin and jump mid air... ,your inventory looks like the old inventory like y'know like a list your username changes to Steve and nobody can join!!!!!!!!!!!!!!!!!!!!!!!!!!!!