session setting problems

[Renlok]

around my site im using loads of sessions to tell the user different things. on every page they work fine execpt one where i think it sets the session but the session has no value. i think it sets it atleast as i use isset() function to check if its set then if it is it sould print out a small image then the value of the session but all it shows is the little image.

the code in question is as follows:

<?php
session_start();
include("/home/renlok/public_html/action/func.php");
include("/home/renlok/public_html/includes/config.php");

//check logged in
if (!isset($_COOKIE['mysite_username']))
{ 
$_SESSION['message'] = "Please login";
header('location: http://www.roeonline.co.uk/'); 
}

// connect to the mysql server
$link = mysql_connect($server, $db_user, $db_pass)
or die ("Could not connect to mysql because ".mysql_error());

// select the database
mysql_select_db($database)
or die ("Could not select database because ".mysql_error());
$messagein = $_POST['message'];
$topic = $_GET['topic'];
//checks if there is a message
$lenght = strlen($messagein);
if($lenght == 0)
{
	echo "It appears you have not entered a message";
	header("Refresh: 2; URL=\"http://www.roeonline.co.uk/forum/topic.php?topic=$topic\"");
	exit;
}
//sort out entered html
$messagehtmlfix = htmlspecialchars($message);
//stick bb code in
$message = bb2html($messagehtmlfix);
   $username = $HTTP_COOKIE_VARS['mysite_username'];
   $query = "SELECT * FROM users WHERE md5='$username'";
   $result = mysql_query($query);
   $row = mysql_fetch_array($result);
   $user = $row['id'];
$query = "insert into forumposts (message, poster, topic) values 
    ('".$message."', '".$user."', '".$topic."')";
$result = mysql_query($query);
//lastupdate checker
$lastupdate=time();


$topicquery = mysql_query("UPDATE forumtopics SET lastupdate='$lastupdate' WHERE id='$topic'");
$topicquery2 = mysql_query("UPDATE forumtopics SET lastposter='$user' WHERE id='$topic'");

//setting event
$points = $row['points'];
//get random points
$x = rand(1,4);
$y = rand(1,4);
$z = rand(2,18);
$postreward = ($x*$y)+$z;
//give user points
$newpoints = $points+$postreward;
//set event session
$_SESSION['event'] = "You earned $postreward points for posting";
$sqlreward = "UPDATE users SET points='$newpoints' WHERE id='$user'";
$runreward = mysql_query($sqlreward) or die($sqlreward.mysql_error());

header("Location: http://www.roeonline.co.uk/forum/topic/$topic.php");
?>

the session im having problems with is $_SESSION['event'] near the bottom, the other $_SESSION['message'] seems to work fine and the 'event' one used to work but stoped working all of a suddun for no reason what so ever.