Hi, I'll try and explain.....
5lb of fat is about the same size as a loaf of bread, and 5lb if lean muscle is about the size of your hand. It's not that muscle weighs more than fat it's just has more density.
In reality, what has probably happened to your body is one of two things. The first is your body is trying to cope with the new eating habbits and has slowed down for a little while. The second thing is if you have been working out then you'll more than likely have lost fat but replaced it with your muscles becoming stonger.
It's difficult because you want to look in the scales and see them going down every time you get on them but it doesn't happen. It helps me to remeber the scales are a tool, they are not the be all and end all of my weight loss and though I haven't lost a pound since April I have been working out a lot and my body has really changed a lot. (even my fingers!!lol).
I hope that helps a wee bit for you, and I'm always here if you need a chat! xx