It's water. I definitely feel puffy after I've worked extra hard with the weights--that's why it's important to take rest days and give your body a chance to recover. This week I moved to higher weights and lower reps in my workout, and I've felt puffy all week--just started to feel a little more normal yesterday, and my jeans were even a little loose! Make sure you are drinking lots of water, getting plenty of protein, and try to keep your sodium down!

I should take my own advice--my sodium has been off the charts this week So, that makes a double whammy on my puffiness--lots of sodium and higher weights--no wonder!

Edited to note that you weren't using weights, you were only doing cardio--any change in intensity can cause water retention, so whether you are lifting weights or doing cardio, this can make you feel a bit puffy for a few days...