To lose 3-5 pounds per week is crazy! That's 10800 - 18000 calories less than used per week. That's 1550 - 2570 calories less per day. A sedentary person needs somewhere between 2000 and 3000 calories per day. You were either eating half as much food than you needed or there is a more likely explanation.

It's obviously a combination of eating less and losing water weight. Your body can maintain about 1 lb of glycogen and 3 lb of water associated with that glycogen. Not eating carbs will likely lead to a big decrease in this weight. Also the fat itself is stored in 1:1 ratio with water. So your 3-5 lbs per week is likely either a 1 or 2 time thing where you progressively lose all your glycogen. Together with a moderate calorie restriction you end up losing some fat and an equivalent amount of water.