Just wanted to chime in and say that carbs/sugar are pretty essential for anyone's metabolism.
Glucose is the main source for energy in your cells and carbs provide all of your "short-term" fuel.
When you don't have enough carbs, you're body turns to fatty acid oxidation, which isn't as efficient.
Long story short, you really should incorporate carbs back into your diet. They reason you lost all that weight is because your body started using your "long term" energy storages to compensate. It's not healthy in the long run.
edit- I'm a 4th year Biochemistry major. <- Adding some credibility to my post