Weight problem? // Jillian Micheals 30 day shred.?

I'm 16, and I weigh 159. I'm 5"5. I lost 6 pounds this week doing the 30 day shred. I've been drinking water, but a lot. My main concern is that losing that much in a week seems unhealthy, from what I've heard anyway. Oh, and I eat around 800-1,000 calories a day. If I am losing water weight, how much water weight will I lose total? My weight goal is 135-140 lbs.