"No tables used" error when calling get() after query() in CodeIgniter

public function getQuestions($params = "", $page = "all", $count = false){
    $this->db->query('SELECT  questions.questions_id, questions.question_description,
            questions.question_explanation, questions.created_date,
            questions.updated_date, questions.is_active,
            diffLevels.difficulty_levels_title
        FROM  '.TBL_QUESTION.' as questions
        INNER JOIN  '.TBL_DIFFICULTY_LEVELS.' as diffLevels
               ON questions.fk_difficulty_levels_id = diffLevels.preference
        WHERE  questions.is_active=1');
    $Q = $this->db->get();
    if ($Q->num_rows() > 0) { 
        foreach ($Q->result() as $row) { 
            $data[] = $row; 
        } 
    } 
    $Q->free_result(); 
    return $data;
}

This is my query ...and I did few many tweaks, but it won't work.

Try this Query:

$this->db->select('
        questions.questions_id,
        questions.question_description,
        questions.question_explanation,
        questions.created_date,
        questions.updated_date,
        questions.is_active,
        diffLevels.difficulty_levels_title 
        ');
        $this->db->from("questions");
        $this->db->join("diffLevels",'questions.fk_difficulty_levels_id = diffLevels.preference' , 'inner');
        $this->db->where("questions.is_active",1);
        $query=$this->db->get();
        $data=$query->result_array(); 
        //echo $this->db->last_query();
        //echo "<hr/>";
        //echo "<pre>";
        //print_r($query);
        //exit;

I have directly used table name as "questions" and "diffLevels",Please change accordingly.