When you begin any new weightlifting program your body will retain water in the beginning. This is because your body's muscles will store more glycogen when you start a new program and this attracts more water to your muscles. This is why your muscles swell. After your body adapts to the new program your body will stop storing excess glycogen and the water retention will decrease.
A story I often tell related to this is about a workout I did with a young employee of ours about 25 years ago. We decided to do the hardest and most insane back workout ever. We did as many sets of pull ups as we could and then did negatives on a Nautilus Chin and dip machine starting with about 240 lbs and worked our way all the way down the weight stack (24 sets! of negatives). DOMS are not something that are desirable, but if you want to get them, do negatives - Ouch!
On the plus side after completing this workout my biceps grew "temporarily" over 1 inch. This was not muscle unfortunately, but water retention and swelling. On the negative side I couldn't lift my arms over my head for at least a month- LOL. Certainly, not the smartest way to work out, but does illustrate in an extreme way how water retention is caused by weightlifting.
Remember, more than half your body's weight is made up of water. Short term fluctuations in your weight have more to do with increases or decreases in water retention - not muscle and/or fat.