Problem with loop in single.php

I’m making a loop (using multiple loops) in order to show the related post from the main article category. The problem is that the loop prints the last posts from the month when I specified a category ID.

Note: I obtain the category ID before and it’s contained on the $categoria variable. If i “echo” the $categoria inside the loop, it shows me the correct ID, but i don’t know why is not working.

The same code works well on index.php but not here:

`<?php $events_query = new WP_Query(“‘cat=”.$categoria.”&posts_per_page=3′”); ?>
                  <?php $postnum = 0;?>
                  <?php if(have_posts()) : ?><?php while($events_query->have_posts()) : $postnum = $postnum + 1; $events_query->the_post(); ?>
                    <?php if ($postnum > 1 ) { ?>
                        <div class=”cuadrorelated”>
                            <div class=”imagenrelated”>
                                <?php if(has_post_thumbnail()) { ?>
                                    <?php the_post_thumbnail(‘posthumb’); ?>
                                <?php } else {?>
                                    <img src=”<?php bloginfo(‘template_directory’); ?>/images/hemeroteca.jpg” />
                                <?php } ?>
                                <h1 class=”negrocentro”>“><?php the_title(); ?></h1>
                            </div>
                        </div>
                    <?php } ?>
                  <?php endwhile; ?>
                  <?php endif; ?>

Thanks