I have doen two MCs in the past and am on Day 5 of my third. In the past I have noticed that I retain a lot of water the first few days and then magically it comes off quickly.
Are you doing the salt flushes? I had a hard time with those, as I retained so much water. Also, I always drink more lemonade at first--when ever I feel hungry and this helps to keep up your spirit and energy. Usually I don´t want to drink as much so I can lose weight but eventually your body asks for less and my day 10, 14 et cetera you can drink 2 lemonades and still have a ton of energy. I think you might have limited yourself too much from the start and your body is responding by going into starvation mode. This is triggered by eating habits as well as mental thoughts. This is why it is important to drink when you need to and not to limit yourself to only 3-5 drinks a day.
As you are already on day 7 of the Master-Cleanse a lot of this can not be changed but I would suggest making this experience more pleasant and productive.
Our bodies are strange--you can intake more calories and lose more weight if your body is happy. If it is not happy you will have difficulties convincing it that it is not going to starve and to burn the existing fuel.