Losing 9lbs per week is ridiculous, no wonder you felt weak.
The worst part is that no more than 1 lb of that was fat. The rest was glycogen and water. That's part of the reason why low-carb diets are so popular. You cut out carbs from the diet, your weight rapidly drops 5-10 pounds, and you start thinking "hey, that diet really works!" Except, of course, it doesn't. Actually losing 9 lbs of fat requires an energy deficit of 30,000 calories, almost impossible to achieve with diet alone in less than a month.
Personally, though, I wouldn't go in for any fancy diets, or exclude any food groups - not even carbs. I'd accept the pain of having to count calories, work out my base metabolic rate (there are calculators on-line) and from there I'd be able to work out the balance between consumption, and exercise, that should enable me to lose a couple of pounds per week. If a couple of pounds per week doesn't sound like much, trust me - what you are trying to do is make changes that you might be able to sustain, and in any event, how good would you feel about yourself if in six months time you'd lost 50lbs?
Forget about sports nutrition. Eat a balanced diet. You aren't entering the Tour de France.
Agreed.