Dieting: how much loss is unhealty?

For the past 12 days I’ve been carefully helping my wife with “the17 day diet” and in that time I have dropped 21 pounds. The last time I lost this kind of weight was during my first round of Chemotherapy! I’m pretty sure it’s not water weight because as a result of my previous illness’ I drink a lot of water during the day. This is my first structured diet that I have really tried to follow. Is this a ludicrous amount that I should doubt my scales? I feel pretty good, except that I am hungry all the time. Is it unhealthy to lose weight too fast?