Wordpress Lightbox close displaying responsive menu

So, I installed WP Lightbox 2 in order to enable lightbox functionality on certain images, more specifically featured image on a custom post using code below:

if ( has_post_thumbnail()) {
     $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'large');

    echo '<div class="product-image"><a href="' . $large_image_url[0] . '"' . ' rel="lightbox">';
    echo get_the_post_thumbnail($id,'large');
    echo '</a></div>'; 
    } 

If you go to this URL and click the thumbnail, seems to work fine (tested with Chrome, Safari and Firefox). http://ift.tt/1YryF6o

The issue I'm having is when closing the lightbox, as soon as I click the "x" to close overlay, the top menu suddenly displays the responsive Nav. I tried to inspect to see whether a common CSS class or ID was being updated by closing, but can't seem to make sense of this. Any help would be greatly appreciated.

via Chebli Mohamed