I am soooo with you all. My 5-y-o DD1 has also recently turned from an angel (though this was a shortlived phase) into Kevin the teenager.
"You never let me do anything!"
"You're not nice" (she doesn't know the words "so unfair" yet but when she learns them - God help us!)
etcetcetc
Up till just recently she and her younger sister (3.5) were as thick as thieves but now she has taken to criticising everything DD2 does. Even making fun of her pronunciation (baby lisp), less-than-perfect colouring skills etc., calling her names, poking and provoking to turn an innocent situation into an all-out battle.
Meltdown every time TV is turned off before bed (age-old routine that has never changed).
I think it must be the age and the time of year (too much out of routine, sugar overload, presents every day for about a week, late bedtimes). Where we live 5-y-os don't go to school so she is in the preschool she has been in for over a year, same teachers, same kids, same hours, etc...
She gets loads of cuddles, loads of praise, but I suspect I take a bit too much notice of the naughtiness, take it too seriously instead of distracting, or even ignoring, etc. so we end up with the negative attention spiral. You need the patience of a saint, don't you? Wish I had it!