Pagination breaks custom sub menu

I've got the following class which allows me to make a specific sub menu.

class Custom_Walker_Nav_Sub_Menu extends Walker_Nav_Menu {

var $found_parents = array();

function start_el(&$output, $item, $depth, $args) {

    global $wp_query;

    //this only works for second level sub navigations
    $parent_item_id = 0;

    $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

    $class_names = $value = '';

    $classes = empty( $item->classes ) ? array() : (array) $item->classes;  

    $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
    $class_names = ' class="' . esc_attr( $class_names ) . '"';


    if ( ($item->menu_item_parent==0) && (strpos($class_names, 'current-menu-parent')) ) { 
        $output.= '<li>';
    }


    // Checks if the current element is in the current selection
    if (strpos($class_names, 'current-menu-item')
        || strpos($class_names, 'current-menu-parent')
        || strpos($class_names, 'current-menu-ancestor')
        || (is_array($this->found_parents) && in_array( $item->menu_item_parent, $this->found_parents )) ) {

        // Keep track of all selected parents
        $this->found_parents[] = $item->ID;

        //check if the item_parent matches the current item_parent
        if($item->menu_item_parent!=$parent_item_id){

            $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

            $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
            $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
            $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
            $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';

            $item_output = $args->before;
            $item_output .= '<a'. $attributes .'>';
            $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $args->link_after;
            $item_output .= '</a>';
            $item_output .= $args->after;

            $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
        }


    }
}

function end_el(&$output, $item, $depth) {
  // Closes only the opened li
    if ( is_array($this->found_parents) && in_array( $item->ID, $this->found_parents ) ) {
        $output .= "</li>\n";
    }
}

function end_lvl(&$output, $depth) {
    $indent = str_repeat("\t", $depth);
    // If the sub-menu is empty, strip the opening tag, else closes it
    if (substr($output, -22)=="<ul class=\"sub-menu\">\n") {
        $output = substr($output, 0, strlen($output)-23);
    } else {
        $output .= "$indent</ul>\n";
    }
}

}

And I use the following function to paginate the posts.

function html5wp_pagination()
{
global $wp_query;
$big = 999999999;
echo paginate_links(array(
    'base' => str_replace($big, '%#%', get_pagenum_link($big)),
    'format' => '?paged=%#%',
    'current' => max(1, get_query_var('paged')),
    'total' => $wp_query->max_num_pages
));
}

The problem is, when I click on a page to go, the page loads but the submenu has only one element.This doesn't happen when I go to other main pages such as contact us or any other. What am I doing wrong here since I'm new to wordpress.