5 year old has turned into a nightmare!!!

[Mammyofonlyone]

Help! My usually well behaved (obviously she can be naughty/stroppy at times but on the whole is good) five year old girl has apparently had some kind of personality transplant and is now just vile, and I don't know what to do about it!
All the things I would normally do (such as ask her why she has done/said something, ask how it makes her feel, how she thinks it makes me feel, taken away treats, rewarded good behaviour, sent her to her room, asked her to explain what happened) just aren't working.
There have been no changes or upheavals that I can think of that could explain things. She is very happy in school and doing well and we have done a variety of fun things with friends throughout the holidays.
She has periods during the day of being nice, then will just become obstinate and flatly refuse to do what she has been asked (eg eat another mouthful of supper, brush teeth, etc). She is whiny, shouty or just makes horrible whiny sounds when I try and talk to her. She even will shout 'no!' at me when I'm asking her to do something and has kicked out at times.
It is making me so sad and is now starting to ruin the holidays that we have all looked forward to for so long as I am having to tell her we can't have days out/meet friends due to her behaviour.
If anyone has experienced anything similar and/or can offer any advice I would really appreciate hearing from you, thank you

Could she be tired OP? I don’t mean to trivialise the behaviour but you particularly mention supper. As for the whiney voice, tell her calmly that you can’t hear her when she talks like that and ask her to use her normal voice, then just ignore or repeat. She’ll soon get the message that whining doesn’t get a satisfactory response.

My 5 y o is being similar. Refusing to do what I ask and laughing / running away

Thanks Jilted. She is in the bath for 6.45-7.00pm ish every night and then straight to bed, which has basically been her bed time since birth, so I do t think she is up too late? I honestly can't think how to explain it, or what to do about it. She has been a bit better today as her Granda (my Dad) has been here but she has been fully occupied the rest of the holidays so far.
User, it's grim isn't it? Where has my nice girl gone? I've been looking forward to the summer holidays for ages and now they are here it's a lottery as to if we will have a good day or a bad one

My five year old, too. Is it a developmental stage, I hope? It's mainly just me he does it to, but he's gone from being a timid but very kind and caring boy to lots of defiance, being know-it-all and calling me silly, talking over people all the time, kicking off into strops whenever he doesn't get his way, demanding attention constantly, being hugely ungrateful and just generally being much less pleasant to be around. I'm trying to treat him just as I always have and going this is a stage/reaction to stress (his two year old brother has been hard work lately and he's starting school tomorrow) but I do worry he's developing an unkind and selfish streak. He's generally been great at nursery but has had a few incidents where staff say he seemed not himself - demanding, rude, and the wrong side of confident. For such an extremely kind and gentle kid, it's so out of character. Or is this his school-age character? Help!

OP I could have wrote ur post in fact I just did in the parenting thread I should probably move it lol. My DD is five too it's exhausting everything is a fight! This is my second child and I don't ever ever remember it being this hard with my DS who is now 12. He was a dream never talked back etc . My daughter seems to also have had a personality transplant it's also been since start of summer I live in Northern Ireland so she's been off from end of June . Far too long to be out of normal routine. She can be quite nasty and the tone of voice she uses to talk to me in has such disrespect in it I am gobsmacked . Every day she barges in at 6.30am starting a row because she knows she's not allowed YouTube that early . Any good suggestions folks ? Thinking of bringing the behaviour sticker chart back .