Google Analytics tagging with post slug for Twitter Tools

I’m trying to add Google Analytics link tagging variables to my URLs before Twitter Tools tweets them out. This is what I’m trying to achieve:

http://example.com/?utm_source=twitter&utm_medium=twitter&utm_campaign=EXAMPLE-POST-SLUG

So, I’ve got this bit of code:

function ses_url_tagger($url) {

	// get the post slug
	$post_data = get_post($post->ID, ARRAY_A);
	$slug = $post_data['post_name'];

	if (stristr($url,'?')) {
		// URL already has query args, tag on the end
		$ses_url = $url . '&';
	} else {
		// URL doesn't have query args, tag on the end
		$ses_url = $url . '?';
	}
	// append slug to URL
	$ses_url = $ses_url . 'utm_source=twitter&utm_medium=twitter&utm_campaign=' . $slug;

        return $ses_url;
}
add_filter('tweet_blog_post_url', 'ses_url_tagger');

This is not working and I guess it’s because the $post_data variable is not available. How do I fix this? Thanks!