problem after switching to %postname%

I am dynamicaly loading submenu with

var newstr="submenu.php?cat=4";
$('#thumb-list').load(newstr);

in submenu.php I have

<?php
$var = $_GET['cat'];
define('WP_USE_THEMES', false);
require('wp-blog-header.php');
...

on “require(‘wp-blog-header.php’);” is where everything breaks
no error, just after calling this nothing works.

Everything was fine before I changed .htaccess to enable pretty links with %postname%

I have no idea why does it happen. Any ideas?