wordpress shortcode appear above the content

Hi i have a custom shortcode, but i really don’t know why my shortcode appear above the content, this is my code:

function paydown_func( $atts ) {
    extract( shortcode_atts( array(
        'id' => '',
        'dialog' => '',
        'header' => '',
        'pay' => '',
    ), $atts ) );

    if ($header == ''){
        $header = 'OPTIONAL<br>DONATION';
    }
    if ($dialog == ''){
        $dialog = 'Don't worry. This resource will always remain free. However I would appreciate if you share some support and help me to get some coffee. Thank you very much!!';
    }
    if ($pay == '') {
        $pay = 'Donate $2';
    }

    $pay_content = '';
    $pay_content .= '<div class="alert-gray">';
    $pay_content .= '<div class="one-tf column-first"><span class="ptitle">'.$header.'</span><br>Via Paypal</div>';
    $pay_content .= '<div class="one-ti column-inner psmall">'.$dialog.'</div>';
    $pay_content .= '<div class="one-tl column-last">';
    $pay_content .= '<a href="http://creativestocks.com/?pfd_checkout='.$id.'" target="_blank" class="button blue small alignright"><span class="button-inner">'.$pay.'</span></a>';
    $pay_content .= '</div></div>';

    return $pay_content;
}