Well they weigh the same--a pound of muscle weighs a pound as does a pound of fat. The difference is the pound of fat will take up a much bigger space since muscle is more dense. There is a great picture on the Internet comparing the two--I am sure someone will post it.
So it is possible, over time, to become leaner while staying the same weight or even gaining a little. The clue would be losing inches, looking more toned and wearing a smaller clothing size. I am not sure how common that is for people with a fair amount to lose though, I imagine it might be more common when close to goal (not sure where you are, just saying...)
I don't think you gained 5 pounds of muscle in two weeks. I really don't think that is possible. Bodybuilders train hard on very focussed programs and eat carefully planned diets and do not gain muscle that quickly. I don't think your gain is fat though either (unless you are getting bigger). Most likely it is increased fluids retained in your body. There are a lot of different things that can cause increased scale weight from fluids so most day to day ups and downs are related to this. When I strength train, I typically weigh about 3 pounds more. If I start after several rest days I see an immediate gain that just appears and if I go long enough without strength training I see a similar loss just overnight (happens on vacation for me frequently--the first time I was confused how I lost weight on vacation when my diet wasn't great). This is one reason why it is also a good idea to take measurements as well as weighing yourself. The scale weight is made up of everything in your body even if just passing through. It can help to have a second method to track progress.
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