Wordpress Echo Post if In Category

function delicious_promocoes($atts, $content = null) {
    extract(shortcode_atts(array(
        "items" => '2',
    ), $atts));

    global $post;

    /* DEBUG STARTS HERE - I want to show only posts on the category "destaque" */
    $categories = get_the_category($post->ID);
    var_dump($categories);
    /* DEBUG ENDS HERE - But the var_dump returns array(0) { }*/

    $retour ='';
    $retour .= '<div class="centered-wrapper">';
    $retour .= '<section>';
    $retour .= '<ul id="promocoes-home-principal" style="height:300px">';

        $args = array(
            'orderby'=> 'post_date',
            'order' => 'rand',
            'post_type' => 'portfolio',
            'posts_per_page' => $items
        );

        $my_query = new WP_Query($args);
        if( $my_query->have_posts() ) {
            while ($my_query->have_posts()) : $my_query->the_post();

            $portf_icon = get_post_meta($post->ID,'dt_portf_icon',true);
            $thumb_id = get_post_thumbnail_id($post->ID);

            $mythumbnail = wp_get_attachment_image_src(get_post_thumbnail_id(), 'portfolio-thumb');

            if ( has_post_thumbnail() ) {
                    $retour .='<li>';
                    $retour .='<a href="'. get_permalink() .'" title="'. get_the_title() .'">';
                    $retour .='<span class="item-on-hover"><span class="hover-link"></span></span>';
                    $retour .='<img src="'. $mythumbnail[0] .'" height="'. $mythumbnail[2] .'" width="'. $mythumbnail[1] .'" alt="'. get_the_title() .'" /></a>';
                    $retour .='<span class="caption"><a href="'. get_permalink() .'">'. get_the_title() .'</a></span>';
                    $retour .='</li>';
            }
            endwhile; wp_reset_query(); }
        $retour .='</ul>';
    $retour .='</section>';
    $retour .='</div><!--end centered-wrapper-->';

        return $retour;

}

add_shortcode("promocoes", "delicious_promocoes");