Next/Previous post of category

Hi all,

I integrated wordpress into one of my websites to use it as a content management system. Now I have the following problem:

There are two categories, CAT1 and CAT2. Both categories contain images and details from art. Now I want to add a PREVIOUS and NEXT button to each post of a category, so you can switch between the different pieces of art.

CAT1 uses index.php

<?php
	$lastposts = get_posts('numberposts=1&offset=0&category=3&orderby=post_date&order=ASC');

	foreach($lastposts as $post) :
    		setup_postdata($post);
		?>

	<?php the_content(); ?>

	<ul>
	<li class="voorbeeld-vorige"><a href="<?php next_post_link('%link', 'Next post in category', TRUE, '13'); ?>"><span>vorige foto</a></li>
	<li class="voorbeeld-volgende"><a href=" <?php previous_post_link('%link', 'Previous in category', TRUE, '13'); ?> "><span>volgende foto</span></a></li>
	</ul>

	<?php endforeach; ?>

But this doesn’t work. It does display the next and previous link, but they don’t link to the next or previous post of the category.

Does anyone know how to fix this problem?

Thanks in advance!