Are you on a low-carb diet?
Are you doing something like sure-slim?
What is your diet?
How long have you been doing this for?
What are your current stats?
Would you be considered obese?
When you lose weight rapidly, the majority of the weight will be water and lean tissue.
The lean tissue may be skeletal muscle or organ tissue, bone.
We would really need a bit more info, but in general, a slower weight loss is preferable.