Like the above poster stressed, don't worry about it.

Muscle is 1/3 as dense as fat, so it won't show as much.

Putting on muscle helps with weight loss, because you burn more calories throughout the day. It costs a lot more energy to maintain 20lbs of muscle than it does 20lbs of fat.

I would suggest going to once per week weighing if seeing fluctuations like this stress you out.