Ajax / jQuery / PHP使用请求设置Cookie

I have a login script that worked last night, made no modifications since then.

Before everyone downvotes, I have already read 10+ similar questions, but they didn't help me out.

I have a login page, with a user and pass field, when users submit, a jQuery event happens where it gets posted to a URL, and that PHP script then checks for the users existence and sets cookies if they do exist.

Here is the jQuery:

$.post('http://*******/login/', {user: $('.login-form input[name=username]').val(), pass: md5($('.login-form input[name=password]').val())}, function(data) {
    if(data.status == 'success') {  
        setTimeout(function() {
            window.location = '/dashboard.php';
        }, 2000);
    } else {
        alert('No valid user found');
    }
});

On the other end of this jQuery, is the PHP, which uses a basic API that I have built:

<?
require_once('../../classes/api.php');

$api = new API;

$username = $_POST['user'];
$password = $_POST['pass'];

if(strlen($password) != 32) {
    header($_SERVER['SERVER_PROTOCOL'] . ' 500 Internal Server Error', true, 500);
    die();
}

$dbCon = $api->db("***");
$checkUser = $api->login($dbCon, "users", $username, $password);

if($checkUser == false) {
    echo json_encode(array('status' => 'error'));
} else {
    $company = $api->getCompany($dbCon, "companies", $checkUser['company_id']);
    setcookie('first_name', $checkUser['first_name'], 0, '/', '.mydomain.com');
    setcookie('last_name', $checkUser['last_name'], 0, '/', '.mydomain.com');
    setcookie('company_id', $checkUser['company_id'], 0, '/', '.mydomain.com');
    setcookie('agency_account', $company['agency_account'], 0, '/', '.mydomain.com');
    setcookie('advertiser_id', $company['advertiser_id'], 0, '/', '.mydomain.com');
    echo json_encode(array('status' => 'success'));
}

?>

When I check the Chrome DevTools network section for the post, it says:

Set-Cookie:first_name=first; path=/; domain=.mydomain.com
Set-Cookie:last_name=last; path=/; domain=.mydomain.com
Set-Cookie:company_id=1; path=/; domain=.mydomain.com
Set-Cookie:agency_account=true; path=/; domain=.mydomain.com
Set-Cookie:advertiser_id=12; path=/; domain=.mydomain.com

But no new cookies are made. What could be causing this now? Like I said, it was working last night, but not today.

I am willing to add and edit the question and my code as needed.