On the Bright Side, I'm Now the Girlfriend of a Sex God by Louise Rennison (2002, Paperback, Reprint)

Georgia Nicolson has started dating the Sex God (aka Robbie). So life should be perfect...except in Georgia's life, nothing is ever perfect. Her cat, Angus (the size of a small Labrador), is terrorizing the neighborhood. Her sister, Libby (who is slightly mad), hides her pooey knickers at the bottom of Georgia's bed. Her mother is clearly devoted to making Georgia's life miserable, and even her best friend, Jas, is "half girl, half turnip."

Then the Sex God breaks it off because she's too young. It's time for a plan. It's time for a Red Herring. It's time for Georgia to become a "heartless boy magnet!"