What you're experiencing is perfectly normal. It's nothing more than a temporary water weight gain. Ever notice your weight goes up during TOM ? Most women tend to gain weight during their menstrual cycle. Is that a fat gain ? Nope, it's nothing more than a temporary water weight gain that passes in a few days.
When you work your muscles intensely (say strength training), your muscles soak up water like a sponge. This is what they are supposed to do. Your muscles will release any excess water they don't need once they've adapted to the new routine.
In short, while a safe weekly weight loss would be 1-2 pounds per week, there may be weeks you don't lose. There may even be weeks you gain ! And that doesn't mean you're doing anything wrong. These types of water fluctuations really are normal.
Women don't pack on 2 pounds of muscle overnight. It's impossible. However, it IS very easy to retain water. I can easily gain or lose 3-4 pounds of water weight in a day. I don't let it freak me out because I know it's a normal fluctuation. If you weigh yourself regularly, you'll notice these fluctuations too.
Nothing to worry about.