Your body's hormonal resopnse to stress includes release of a cocktail of adrenal hormones that, in combination, cause what you're experiencing:

-Cortisol: A catabolic (breaks stuff down) hormone that, during times of stress, seems to act preferentially on the protein (muscle) in your body. Normally, this would be released during exercise and would aid in the breakdown of fat and (to some degree) stored glycogen as well as a small amount of muscle in order to keep up with ATP production.

-Epinephrine: One of the most multi-tasking hormones in the body. It's not only catabolic to fat and glycogen, it is also a powerful stimulant to the body. Excess release will result in a reduced appetite, higher resting heart rate & blood pressure, and elevated body temperature. It's mainly the stimulant properties that will cause weight loss.

I'm better at science than I am at psychology, but hopefully this helps...