Form in plugin redirects to 404 page

Hey, I’m writing a plugin that has a shortcode that displays a form. But when I submit the form it redirects to the 404.php page. What am I doing wrong?

Here is my code:

function ShowAddTestimonialForm()
{
    global $wpdb;

    require_once('recaptchalib.php');
    $publicKey = '6LevfbwSAAAAAI-IQCL4v1EFEA6KIZbI7g4uDpkq';
    $privateKey = '6LevfbwSAAAAAHFqkdyJqsXR5aPU_-085X6njYVk';

    if (isset($_POST['submit']))
    {
        if (!empty($_POST['name']) && !empty($_POST['email']) && !empty($_POST['testimonial']))
        {
            $resp = recaptcha_check_answer($privatekey, $_SERVER['REMOTE_ADDR'], $_POST['recaptcha_challenge_field'], $_POST['recaptcha_response_field']);

            if ($resp->is_valid)
            {
                $dataToInsert = array(
                    'Name' => $_POST['name'],
                    'Email' => $_POST['email'],
                    'Testimonial' => $_POST['testimonial'],
                );

                $wpdb->insert(WP_TESTIMONIAL_TABLE, $dataToInsert);

                $success =  "<p style='text-align:center;'>Thank you for adding your testimonial!</p>";
            }
            else
            {
                $error = "The code you entered is not correct. Please try again.";
            }
        }
        else
        {
            $error = "Please make sure all required fields are not left blank";
        }
    }

    ?>
        <?php if (!empty($success)) { echo $success; } else { ?>
        <form name="testimonialForm" method="post" action="<?php echo str_replace( '%7E', '~', $_SERVER['REQUEST_URI']); ?>">
        <p style="text-align:center;"><strong>All fields with an asterisk (*) indicate a required field.</strong></p>
        <p style="text-align:center; color:red"><strong><?php echo $error; ?></strong></p>
            <table>
                <tr>
                    <td><label for="name">Name (*):</label></td>
                    <td><input type="text" size="20" id="name" name="name" required></td>
                </tr>
                <tr>
                    <td><label for="email">Email: (*)</label></td>
                    <td><input type="email" size="20" id="email" name="email" required></td>
                </tr>
                <tr>
                    <td><label for="testimonial">Testimonial (*):</label></td>
                    <td><textarea id="testimonial" name="testimonial" rows="5" cols="30"></textarea></td>
                </tr>
            </table>
            <div style="clear: both; height: 1px;">&nbsp;</div>
            <?php echo recaptcha_get_html($publicKey); ?>
            <input type="submit" name="submit" value="Submit" />
        </form>
        <?php } ?>
    <?php
}

Can anyone help? Thanks!