The muscle cells in the body are lean and long cells. Fat cells are opposite. When you engage muscles in a workout, especially one that works the muscles as with "strength training" you tend to develop those muscle cells more, and muscle cells are more dense - so they weigh more. So a few good rules of thumb if you are trying to "lose weight" :
Don't look at the scale!!! It's not the numbers on the scale that you want to be watching to drop. Its the amount or percentage of body fat you have that you want to shrink. There are a few ways to do this.
----- You can get some kind of device to measure your body fat like this one listed with Amazon. This uses the skinfold method -- which is the most common. Which means that you pinch your skin and with the device it can tell you your percentage of body fat based on your weight too. And through hospitals and such you can get more accurate tests done but they can become quite costly.
Here is another great website that you can check out just to get some answers to some of your "questions" about weightloss.
MyFitnessTrainer -- the site is owned and operated by one of my friends from college and she's good at what she does. I took fitness for life back in college and have been very athletic most of my life, until about 10 years ago, when I only get on and off workouts as time and children permit. One of these years when my kids aren't so "helpless" and I'm not so "financially challenged" I think that I will get a membership to a gym, and what I wouldn't do to play Basketball, Volleyball and maybe Softball again. I played all three in high school, and loved almost every minute of it, I was in such great shape and I was working with weights and I felt really good back then. But of course, I've been a SLACKER lately and know I need to do something about it, but my kids just cling to my legs if I try to do anything at home, and well, I could just come up with a million and one more excuse of why I don't, but I'll not waste my precious time on that and instead do something more productive like "workout".