Why does this session always return true?

I'm having trouble with my code. Somehow an if-statement that checks sessions always returns true. Even at start. I can't really figure out why..

Here is my login page.

<?php
session_start();
if(isset($_SESSION['username']))   //<-----This always returns true here for a strange reason.
{
}   
?>

Here is my my login controller.

if($_POST)
{
    if(isset($_POST['submit']) AND $_POST['submit'] == "login")
    {
        $username = $_POST['username'];
        $password = $_POST['password'];
        try
        {
            include '../model/Login.php';
            $login = new Login($username, $password);

            if($login == TRUE)
            {
                session_start();
                $_SESSION['username'] = $username;
                header("Location:../index.php");
            }

        }
        catch (Exception $exc)
        {
            echo $exc->getMessage();
        }
    }
}