How come I lost weight with ON Serious Mass?

So I'm a skinny teenager and I wanted to gain some weight so I bought Optimum Nutrition's Serious Mass. I took 1 serving yesterday and half a serving earlier today. I also ate like I usually do. After I took my half serving today I was curious to see if maybe I would gain a pound or something. I look and see that I lost 5kg from yesterday! How is this possible? If anyone can help me out I would appreciate it.