this will show up in every single post (previous and new posts) made by the user, which would be pretty annoying. and you should change the location of where you've put your "custom code"
Code:
$post[fieldX] <br />
Should go before the $post[message], and not outside of the div for it like so:
Code:
<div id="post_message_$post[postid]" align="left">$post[fieldX] <br />$post[message]