What you're experiencing is perfectly normal. It's nothing more than water retention. When you work your muscles intensely, your muscle fibers 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 recovered from the activity. You may also notice your muscles swell in size because of water retention after a workout too. Don't let this scare you.
And it's not just strength training that can cause water retention in muscles, you may notice this after an intense cardio workout or when you change your routine. www.sparkpeople.com/community/ask_the_expe