session problem

i have problem in sessions.

PHP Code:

$this->obj->session->set_userdata("Auth", "Y");
                $this->obj->session->set_userdata("UserName", $Userdetails['UserName']);
                $this->obj->session->set_userdata("UserFullName", $Userdetails['FullName']);
                $this->obj->session->set_userdata("UserId", $Userdetails['ID']);
                $this->obj->session->set_userdata("UserType", $Userdetails['Status']);
                $this->obj->session->set_userdata("UserRoleID", $Userdetails['RoleID']);
                $this->obj->session->set_userdata("UserRoleName", $Userdetails['RoleName']);
                $_SESSION = $this->obj->session->userdata; 


The above code i am using for session init. Sometimes automatically each page asks for login i don't know what happen. Once i clear cookies and recent history in my browser it works fine. May i know what is the problem and also i have give below the logout code.

PHP Code:

$this->session->sess_destroy();
        session_unset();
        session_destroy();
        $this->session->set_userdata("Auth", "Y");
        $this->session->set_userdata("UserName", null);
        $this->session->set_userdata("UserFullName", null);
        $this->session->set_userdata("UserId", null);
        $this->session->set_userdata("UserType", null);
        $this->session->set_userdata("UserRoleID", null);
        $this->session->set_userdata("UserRoleName", null);

        $this->auth->Authencation("Y", __CLASS__);