From the internet:

IF YOU'RE JUST starting out on a weight-training regimen, don't be surprised if you happen to gain three or four pounds in the first few days. No, this doesn't mean that you'll look like you have a gland problem by the end of the week. Inflammation can make muscles enlarge and swell, trapping water in the muscles, says William Kraemer, Ph.D., an exercise scientist and professor at the University of Connecticut, in the Georgia Tech Sports Medicine & Performance Newsletter

just got your link in my last post mandalinn! Thanks!! Very, very helpful!!!!!!!!!!

Anisk, thanks for the info! I remember sitting at a Weight Watchers meeting and the leader talking about myths and telling us that the saying about muscle weighing more than fat is a myth. I had started the week before a strengthening class and was feeling okay that I had actually gained a pound. When she said that I felt so depressed because the instructor for the strengthening class told me that for a few weeks I would actually gain weight because of using small weights and resistance bands. It's good to know that my weight gain could have been from water weight. Now when I go back to the class and gain a pound or two I won't feel so much like giving up! This is good to know!

It's not that it's a myth - muscle, by volume, is heavier than fat. That is, if you are 150 lbs and very muscular, you are going to be smaller than someone of the same height and weight with, say, 30% body fat. But I'm pretty sure takes a lot longer than a week to gain a whole pound of muscle! Good to know we were right about the water-weight gain!