PHP Session data lost on first reload with jquery load() - php

Question

I'm loading content into a div with jquery's load() function and refresh that div every 15 seconds (it checks for a status).
For some reason after the first reload / refresh of the div, the session data is lost.
The exact code I'm using is very long, so a basic version of what I'm doing is this:
load.php
<?php
session_start();
$_SESSION['testvalue'] = "TESTVALUE";
?>
<div id="loaddiv"></div>
<script src="js/jquery.js"></script>
<script>
(function($)
{
$(document).ready(function()
{
$.ajaxSetup(
{
cache: false,
});
var $container = $("#loaddiv");
$container.load("load_test.php");
var refreshId = setInterval(function()
{
$container.load('load_test.php');
}, 10000);
});
})(jQuery);
</script>
load_test.php
<?php
session_start();
echo $_SESSION['testvalue'];
?>
Works fine when page loads initally and displays TESTVALUE, after the div refreshs there is no more output however.
Anyone experienced this before and knows what's going on ?
//EDIT
I'm one step further to solving this:
On the main page I'm selecting some things from my mysql db and putting this into the session, in the subpage (the one that jquery load() loads ) I am now just outputting the session values.
There's absolutely nothing on the subpage but this:
<?php
session_start();
echo $_SESSION['test_one'];
echo $_SESSION['test_two'];
?>
<?php echo session_id(); ?>
When jquery load() is triggered the second time, the values in
$_SESSION['test_one']; and $_SESSION['test_two']; change ... (It is running the mysql query again and putting something else into the session)
The values for the session get set on the mainpage, so why would refreshing the subpage run the mysql query on the main page again ?!