How much water weight is lost is very dependent on your caloric intake, how much water you drink and the amount of time it took. The likely hood that the first ten pounds was indeed all fat loss is highly unlikely, but about 3 pounds is probably more accurate. Here is where it gets boring... First figure out you healthy daily caloric intake should be, say 2500 cal. Then figure out how many calories you ate everyday, say 1500 or so. Multiply 2500 by the number of days (we will say 14 days) it took you to lose that weight. That gives you 35,000. Then multiply 1500 by 14 as well. That will give you 21,000. 35,000 - 21,000 = 14,000. There is 3500 calories in 1 lb of fat so the total non water weight loss would be 4lbs. Feel free to say "wut" or tl;dr after that rant.
I used to do this to gauge whether I was actually losing weight, gaining muscle or just shedding water weight...
I never work out in heavy clothing, it just forces more sweat and cuts water weight. That's just for wrestlers and boxers to make weight.
