It's been my experience that weight loss can plateau for several days in a row, but your case is exceptional. It reminds me of yogis who can survive for very long times without eating anything at all. Here is a link to a documentary about a youth in Nepal who has meditated under a tree for ten months without food or water, believe it or not: http://bit.ly/176IiN3
I think it is possible that the body can enter a state of hibernation. There is another explanation for your case, though. You may be burning calories but adding water. A gallon weighs eight pounds which could mask a week. Try drinking pure water only when thirsty. Or maybe the explanation is that your scale is wrong. Try a different scale.