wp_insert_post_data, admin_notices strange behaviour

function notices(){
 if(!empty($_SESSION['notices'])) print  $_SESSION['notices'];
   unset ($_SESSION['notices']);}
add_action( 'admin_notices', 'notices' );

add_filter('wp_insert_post_data', 'processdata', '99', 2);
function processdata($data) {
$processed_content = filterthis($data['post_content']);
if($processed_content!='error') {
$data['post_content'] = $processed_content;
$_SESSION['notices'] .= '<div class="updated"><p>Everything is ok</p></div>';
return $data;
} else {
$_SESSION['notices'] .= '<div class="error"><p>Something went wrong</p></div>';
return $data;}