When energy balance in the body is positive, body mass decreases?

This is false. This is because a positive energy balance denotes an excess in caloric intake beyond the basal metabolic rate. Excess calories is stored as fat and this can cause increase in body mass. The correct answer is that a negative energy balance will cause the body mass to decrease as caloric intake is below the basal metabolic rate and therefore energy should be derived from endogenous sources such as glycogen or fat tissue. 

You should never redesign your experiment. try and see what in your experiment you made a mistake on or try and test your experimemt with somebody else. see if they get the right results.