Cat and the Stinkwater War by Kate Saunders
At first glance there's nothing special about the little white stone which Cat's archaeologist father gives her. But then Cat makes a catastrophic discovery. The stone does have amazing powers. Power to take her away from bullying Emily Baines. Power to turn her into someone graceful, fast and agile. Power to turn her into a CAT!
But all is not well in the feline world, for the cats' symbol of nationhood -- the Blessed Sardine -- has gone missing. And the only one who can find it is a small marmalade cat called Cat!