postback not working

I have created a form that displays a form on load and when the user clicks the “submit” button using get method the form postsback to itself and is supposed to add a record to the DB.

the page loads up fine hyowever when you postback a “page not found” error appears.

anyone know why?

Page code

<?php get_header(); ?>
<?php 

/*
 Template Name: Tag Connect
 */
global $wpdb;
if (isset( $_GET["flag"]))
{

$catid = $_GET["category"];
$tagid = $_GET["tag"];

$query = "insert into wp_tag2cat (cat_id, tag_id) values(".$catid.",".$tagid.")";
echo $query;
$myrows = $wpdb->query( $query );
}else{
list_cat_tag();

}
?>
<?php get_footer(); ?>