My biggest losses were the weeks where I spent an hour and a half in the gym 4 times a week! I lost 6 and 7lbs, whereas with not exercising as much I was only averaging around 2/3lbs. But I made sure to drink an extra half litre of water for every 30 mins of exercise I did to replace the water your body sweats out, which is why it affects some losses.
Building muscle won't really become a big factor in affecting weight loss unless it is on a very large scale, bodybuilding for example. General toning and cardio does not increase muscle mass enough for it to show up on the scales, Hope this helps hun =D xxx