MOD_REWRITE & PHP $_SERVER['REQUEST_URI'] ISSUE - php

Question

** UPDATED EXAMPLE **
For example, lets say my website url is "http://www.mydomain.com".
The site uses $_GET variables to determine pages, content etc.
So, let's say i'm on http://www.mydomain.com/cms/index.php?p=about-us
This would be the about us page within the CMS.
If I echo out $_SERVER['SERVER_NAME'] here, I get "http://www.mydomain.com"
If I echo out $_SEVER['REQUEST_URI'], I get "cms/index.php?p=about-us".
So, with the code below, the "$lastPage" gives me:
"http://www.mydomain.com/cms/index.php?p=about-us", this is exactly what I want.
BUT, as soon as I submit a form, or update content, etc, i'm being redirected to a css file, or an image path.
This ONLY happens when I have the rewrite rule in the htaccess file, which I need in order to have nicely formatted urls.
ORIGINAL QUESTION
I am running a website on a CMS, and within the CMS there is a "last page" variable. This variable is created like this:
$lastPage="";
$lastPage .= $_SERVER["SERVER_NAME"];
$lastPage .= $_SERVER["REQUEST_URI"];
$_SESSION['theLastPage'] = $lastPage;
The variable is used to redirect users to the last page they were on after updating content, submitting forms etc.
It is used like this:
<script>window.location="<?php echo $_SESSION['theLastPage']; ?>"</script>
This works exactly how I want it to...
Until I add this rewrite rule in my htaccess file:
RewriteRule ^([^/]+)\.html$ index.php?p=$1 [L]
As soon as that line is in my htaccess file, the redirect variable breaks, and directs the user to a css file, or image. The rewrite rule works as it should, and it is there in order to use nicely formatted urls throughout the website.
I've been trying to troubleshoot this for the last few days but have had no success.
If anyone could shed some light onto this for me, that'd be awesome.
Thanks!
HTACCESS FILE **
I can't explain the issue fully, because I don't understand why it's happening.
Here's my current htaccess file, there are a few different rewrite rules here, but the first one is the one that's causing the problem.
Options -Indexes
Options -MultiViews +FollowSymLinks
RewriteCond %{HTTP_HOST} !^www.yoursafetyexperts.com$
RewriteRule ^(.*)$ http://www.yoursafetyexperts.com/$1 [R=301]
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_URI} ^/cms/
RewriteRule ^(.+) - [PT,L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)\.html$ index.php?p=$1 [L] // If I remove this rule, my redirect works properly
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/([^/.]+)/([^/.]+)\.php$ index.php?p=$1&post_id=$2&post=$3 [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([^/]+)/([^/.]+)/([^/.]+)/$ index.php?p=$1&catId=$2&catName=$3 [L]
</IfModule>