When you say "cardio", do you mean an endurance activity (like long-distance running or cycling)? The reason I ask is that I think the answer might have to do with glycogen storage and depletion. In short, when you digest carbohydrates they get converted to glucose. If you don't need the glucose right away for energy, it gets stored in your liver and muscles in the form of glycogen (on a side note: your body can only store a limited amount of glycogen; when your stores are full those extra carbs end up being converted to fat). When you exercise for an extended period of time (or if you intensify your exercise routine), you deplete your blood glucose as well as the glycogen stored in your muscles because you need it for energy. When you run out of glycogen, you "bonk" or "hit the wall" -- not a good feeling! After your workout, your body will want to replenish its glycogen stores and will begin converting carbs as described above. However, with glycogen storage goes hand in hand with water retention. This is because glycogen binds to water as it is stored in your muscles. For every gram of glycogen stored, 3-4 grams of water come along with it. Coincidentally, this process is also at play when you see people losing large amounts of "water weight" (think week one of Biggest Loser) when beginning to diet and exercise.
From everything I've heard, this type of weight fluctuation is pretty normal. Oftentimes, the day after a particularly intense workout I will be up a couple of pounds and things will normalize the next day. If you are fairly new to exercise, you may be depleting your stores faster than someone who is more conditioned. I remember this being discussed a while back on the forums (maybe by tandoorichicken??), but I am unsure of which category it was in. In any case, I wouldn't worry about it -- keep drinking lots of water, refeed yourself with complex carbs after a workout, and if you're doing an intense workout form more than 90 minutes (approximately) bring a source of carbs like Shot Bloks or Gatorade to help replenish energy mid-workout.
Keep at it and DO NOT get discouraged by the scale!