The standard replies to the question are Im fine , im ok , im good , im alright etc -- used when you reply, out of good manners., to the question which is made casually and also out of good manners , without expecting you to elaborate .
But , if the person is close , and you wish to tell some truth, even if you answer in the negative ,it is totally ok . Then , maybe , you say : Im fine but i m feeling very tired .. Im fine except for the fact that im so tired because of work i have being doing .lately