Muscle inflammation perhaps? I know when we overwork our muscles especially with weight training that they become inflamed and need to heal.
I had the same problem a few days ago when I did a toning class with free weights. I was sore the next day (and day after that) and felt tender and like I had gained weight. Sometimes you can feel the soreness a few days after a hard workout which can be a reason for your puffy legs. Just some ideas. I'm sure you didn't gain weight from exercising. Drink more water and take it easy and then check the scale.