need to retain nextpage paginating tag in custom query

I need to use $post->post_content to get the raw post so that i can use the EXPLODE php command. But when i do use $post->post_content, it filters out the tags that are in my post which need to be retained. here is my script. what am i doing wrong? i’m just trying to inject an Ad after paragraphs that contain an image in single.php. thanks!

<?php

$content = apply_filters('the_content', $post->post_content);

$contents = array_filter(explode("</p>", $content));

foreach ($contents as $content) {
    if (strpos($content, '<img') !== false ) {
        echo $content;
        echo "</p>after image ad";
    } else {
        echo $content;
        echo "</p>";
    }
}

?>