Help with Shortcode Functionality

Shortcodes should return a string, not echo data… so you have one of two options here:

• Change all echo statements and all functions that ‘echo’ content to use with return (concatenate all on a string, then return it)
• Use output buffering

For output buffering, your code would end up like:

function upcoming_events_shortcode() {
  $args = array( 'post_type' => 'upcoming_event', 'posts_per_page' => 3 );
  $query = new WP_Query( $args );
  ob_start();
  while ( $query->have_posts() ) : $query->the_post();
        echo '<div class="upcomingevents">';
        echo '<h4>';
        the_title();
        echo '</h4>';
        echo '<div class="upcomingeventimage">';
        the_post_thumbnail('medium');
        echo '</div>';
        echo '<div class="upcoming-event-entry-content">';
        the_content();
        echo '</div>';
        echo '</div>';
  endwhile;
 wp_reset_postdata();
 return ob_get_clean();
}

That way, all the echo’ed content will be captured by the output buffer.