I've been noticing that after I do a hard workout (such as say, mile repeats), the next day I'm down a few pounds. Is this just water weight? I eat pretty bad (too much junk food and I'm trying to change that) and I noticed one day I was up a few pounds.. then 2 days alter I ran a hard workout and I was down 5 pounds. Is this fluctuation normal?
It probably doesn't even have to do with the workout. Minor changes like what you wore, time of day, if you ate or drank before hand all affect your weight. I once was in a situation where I weighed myself after dinner, went for a run, took a crap and was about to take a shower and the loss of weight ended up being around five pounds.
Water weight, dumpage, food in the system, other minor difference really add up.
It certainly is not fat or muscle, if that is what you were wondering.
Harder workouts use glycogen more as a fuel - every 1g of glycogen stored also requires 3g of water to be stored, so harder workouts that significantly deplete your glycogen can have a sizeable impact on your weight the next day, especially if you eat poorly and don't refuel properly.
Yes, water weight. Yes, fluctuation is normal.