Much of your weight loss is likely in water, but it is also certainly real weight. The question is which is which. You should always weigh yourself at the same time each day for an accurate picture. Typically 2-3 lbs/week is okay, and more is generally not good. It's not just fat, but muscle breakdown, dehydration, etc. It's generally not good to lose too much too fast, and there are complications that could result, including kidney failure (see rhabdomyolosis). This happened to a patient of mine a few months back...just too much exercise, not enough water, and a young healthy guy ends up with kidney failure, though it was fortunately reversed in his case. It is best to keep yourself hydrated, limit calories, and exercise a reasonable amount. Don't overdo it. I actually was looking at this page the other day, and it is useful. Check it out.

http://www.webmd.com/diet/lose-weight-fast-how-to-do-it-safely