Login Session Issue in Codeigniter

I create login form with multi level function, those code for multi level are work but i got problem on session, when i've login to my account as an admin and direct to admin dashboard but i still can use back page which directly to login page again, so what i've missed?

my login controller:

<?php
defined('BASEPATH') or exit('No direct script access allowed');

class Login extends CI_Controller
{
    function __construct()
    {
        parent::__construct();
        $this->load->model('login_model');
        $this->load->library('session');
        $this->load->helper('url');
        // $this->load->library('form_validation');
    }

    function index()
    {

            $datatitle['title'] = 'Login LPPM UTY';
            $this->load->view('auth/tamplate/auth_header', $datatitle);
            $this->load->view('auth/login_v');
            $this->load->view('auth/tamplate/auth_footer');

    }

    function auth()
    {
        $username = htmlspecialchars($this->input->post('username', TRUE), ENT_QUOTES);
        $password = htmlspecialchars($this->input->post('password', TRUE), ENT_QUOTES);
        $cek_admin = $this->login_model->auth_admin($username,$password);

        if ($cek_admin->num_rows() > 0) { //jika login sebagai admin
            $data = $cek_admin->row_array();
            $this->session->set_userdata('masuk', TRUE);
            if ($data['level'] == '1') 
            { //Akses admin

                $this->session->set_userdata('akses', '1');
                $this->session->set_userdata('user', $data['username']);
                $this->session->set_userdata('name', $data['name']);
                redirect('admin');
            } else { // jika username dan password tidak ditemukan atau salah
                $url = base_url();
                echo $this->session->set_flashdata('notif', '<div class="alert 
                    alert-danger" role="alert">Username Atau Password Salah
                    </div>');
                redirect($url);
            }
        } else { //jika login sebagai dosen
            $cek_dosen = $this->login_model->auth_dosen($username, $password);
            if ($cek_dosen->num_rows() > 0) {
                $data = $cek_dosen->row_array();
                $this->session->set_userdata('masuk', TRUE);
                ($data['level'] == '2');
                $this->session->set_userdata('akses', '2');
                $this->session->set_userdata('user', $data['username']);
                $this->session->set_userdata('name', $data['name']);
                redirect('dosen');
            } else {  // jika username dan password tidak ditemukan atau salah
                $url = base_url();
                echo $this->session->set_flashdata('notif', '<div class="alert 
                    alert-danger" role="alert">Username Atau Password Salah
                    </div>');
                redirect($url);
            }
        }
    }

      function logout(){
          $this->session->sess_destroy();
          redirect('login');
      }
}

Model login_model:

<?php
defined('BASEPATH') or exit('No direct script access allowed');

class Login_model extends CI_Model
{
    function auth_admin($username, $password)
    {
        $query = $this->db->query("SELECT * FROM admin_login WHERE username='$username' AND password=md5('$password') LIMIT 1");
        return $query;
    }

    //cek username dan password pengajar
    function auth_dosen($username, $password)
    {
        $query = $this->db->query("SELECT * FROM dosen_login WHERE username='$username' AND password=md5('$password') LIMIT 1");
        return $query;
    }

}

Admin Controller:

<?php
defined('BASEPATH') or exit('No direct script access allowed');

class Login_model extends CI_Model
{
    function auth_admin($username, $password)
    {
        $query = $this->db->query("SELECT * FROM admin_login WHERE username='$username' AND password=md5('$password') LIMIT 1");
        return $query;
    }

    //cek username dan password pengajar
    function auth_dosen($username, $password)
    {
        $query = $this->db->query("SELECT * FROM dosen_login WHERE username='$username' AND password=md5('$password') LIMIT 1");
        return $query;
    }

}