always weigh in the AM, after you go to the bathroom and before you drink anything - however, remember, a woman's body weight can change from day to day by around 9 lbs just based on water retention.
Muscle mass the same size as a fat mass does weigh more. 10 lbs of fat takes up more room than 10 lbs of muscle.
Don't weigh every day. I'm pushing myself to actually do away with my scale for the first month. Its hard, but that's what I'm wanting to do!