You are not alone!!
My daughter who has just turned 9 yrs, has been mouthy for about a year now at home, she is very good at school, works hard, helps out etc, but at home she moans if asked to do anything and gets very mouthy. Even got to the stage where she will just walk off outside and play with friends.
I have been told its growing stage she is going through, I hope it doesn't last long!!