I lost 8 pounds in a week?

Whenever I start dieting, I loose A LOT of weight the first week. Even if I don't exercise! I eat breakfast lunch and dinner, with snacks in between, and I probably eat about 1200 calories. My metabolism is high I guess, but I also have a lot of muscle on me. I weighed about 160 when I started, and a week later I weigh 152, without any exercise at all. Im not a huge water drinker either, I probably drink around 4 glasses of water a day. Is this normal?