I have trouble with my SESSION that change when i insert somethnig with POST

I have some trouble with code below, what does is to put $level in a $_SESSION and i don't know how it do that. This $level replace my user $_SESSION['level'] and then he is logged out.

<?php
session_start();

if( $_POST['action'] == 'Add' ) {

    $level = $_POST['level'];

?>

<form action="<?php $_SERVER[PHP_SELF]; ?>" method="post" >

    <input type="text" name="level" value="" placeholder="Level" /><br />

    <input type="hidden" name="action" value="Add" />
    <input type="submit" value="Add user" />
</form>