Let $X$ be a scheme and $S$ be sheaf of sets over the fppf topology of $X$. Let $G$ be a group scheme over $X$ and there is an action of $G$ on $X$. Now, I want to look at the quotient $G \setminus X$. The way I want to define quotient is $ (G\setminus S)(U) = G(U) \setminus S(U)
$? Is it necessary that $(G\setminus S)$ is a sheaf? 

Now, suppose $(G\setminus S)$ is a sheaf and is representable. Does that imply $S$ is representable?