Dd is 7, almost 8. Usually very polite and well-behaved, although she does have a somewhat 'sneaky' side (she takes her role as a Slytherin very seriously). She will hide things if she thinks she'll get into trouble. But lately she's swinging from her usual very sweet self to being very rude and cheeky. I am disciplining her for this every time but today has honestly been so draining, swinging from having a great laugh to her being incredibly cheeky and acting like a teenager. She's also started lying about stupid things and I've spoken to her about why this is a huge issue with regards to trust and respect. Is it a 'normal' stage that kids this age go through or is she just behaving in an unacceptable manner? I have spoken to her many times calmly and gently asking her if anyone has said or done anything to upset her or I should know about and she's adamant that no one has. She's very popular at school and academically is gifted/advanced for her age- not to a crazy amount, but 4.5 years ahead in reading/comprehension and above average for maths. We're very proud of her but right now, it's like she's a shadow of herself at times and I just want my daughter back to her happy self 
it's not just me she's doing it to, it's DH and her younger brother as well. Really is teenager-like behaviour.