Problemas de sesion

[alex_rope]

Hola tengo este codigo:

<?php
//captura de datos
$user=$_REQUEST["username"];
$pass=$_REQUEST["pass"];
//coneccion a la base de datos.
$con=mysql_connect("localhost", "root", "");
$query="SELECT * FROM inventariodb.users
WHERE username='".$user."';";
$x=mysql_query($query);
@$user1=mysql_result($x, 0, 'username');
@$pass1=mysql_result($x, 0, 'pass');
@$rol=mysql_result($x,0,'rol');
mysql_close();
if($user==$user1 && $pass==$pass1 && $user!="" && $rol=="admin"){
session_start();
$_SESSION["ok"]=true;
$_SESSION["user"]=$user;
$_SESSION["admin"]=true;


header("Location: /paginas/admin/index admin.php");
exit;
}elseif($user==$user1 && $pass==$pass1 && $user!="" && $rol=="user"){
session_start();
$_SESSION["ok"]=true;
$_SESSION["user"]=$user;



header("Location: /paginas/index.php");
exit;
}else{
session_destroy();
header("Location: /paginas/Iniciar Sesion.php");
exit;
}
?>


A nivel local no me genera errores, pero cuando lo subo al hosting me sale esto:

Warning: session_destroy() [function.session-destroy]: Trying to destroy uninitialized session in

Warning: Cannot modify header information - headers already sent by

que puede ser??