The short answer is: I don't know. I am still quite new to the low carb way of life. Plenty of people have lost a lot of weight this way, and keep it off, so I know it works, but I may need to do some fine tuning.
The long answer is: There are several possibilities. First of all, my weight loss experience on Weight Watchers has taught me that you simply don't lose weight every week. There are random fluctuations at times, and a weight loss plateau is somewhat common. Perhaps the same is true for low carb diets (LCDs).
Secondly, the proportion of fat, protein, and carbohydrates are different for everyone, but I have learned that this is NOT a high protein diet. It is a high fat diet. Perhaps I have eaten too much protein and not enough fat. A lot of people are suggesting that 75% of calories on an LCD should come from fat. In an LCD, when you restrict carbs, you replace them with either fat or protein. I am consciously trying to consume more fat this week and a bit less protein. I have not eaten too many carbs, that's for sure.
Another hypothesis is that I have simply eaten too much, period. I am trying to listen to my body more and pay attention to when I am full. In general, I do not get hungry between meals, but I may be consuming too much at meals. This goes hand in hand with eating too much protein and not enough fat.
Finally, perhaps my body is simply catching up to the rather dramatic weight loss I had during the first three weeks and will continue the downward trend this next week. We will see. One thing is certain, I am committed to this way of life. I have read too many studies now to discount what I have learned about the dangers of carbohydrates to someone like me. I am very sensitive to them.