I have a header.php file that contains link to the css file...
<link rel="stylesheet" type="text/css" href="style.css">
<h3>Welcome <?php echo $login_session; ?></h3>
<td><?php include("categories.php"); ?></td>
<td><?php include("feed.php"); ?></td>
This could be to do with Varnish Cache, which is used to boost loading times of your website by caching assets (such as your stylesheet).
To disable Varnish Cache, add this line to your
Header set Cache-Control "max-age=0, private, no-cache, no-store, must-revalidate"