I can’t believe that you can lose weight that fast, it could not be body fat, must be water. No wonder you would be constipated and tired you bloody dehydrated that’s why.
Your liver and Kidneys must be working over time and the toxins in your body will only be flushed out with plenty of water.
I would think if you can shift a stone in two weeks then go off the diet and go to a balanced food regime with an exercise programme.
These diets play havoc with your organs and metabolism and the long-term side effects far out weight the short term. Guaranteed to eventually put on more weight than you started with, as you metabolism will just go into starvation mode.
For me, energy in should be equal to energy out, this equals no stored energy/fat.
Too much intake and not enough output, is as bad as too much output and not enough intake!
Balanced, controlled method is the way otherwise you will yo yo diet and eventually beat your liver and kidney's into submission.