Stress can most definitely cause negative composition changes within the body. When you are under high levels of stress, your body is going to operate in ways that will ensure its survival. One way of ensuring its survival is by increasing its fat stores. The body does this because it can rely on these fat stores in times of famish. Stress initiates a cascade of physiological events that produce a spider web of affects. One of these physiological events is the increase in the bodies production of the hormone cortisol. Excessive, elevated concentrations of cortisol cause a chain of catabolic based events that ultimately lead to muscle breakdown and an increase in fat deposits.
Not only can stress cause increases in belly fat due to physiological reasons but it can also do it via psychological reasons. If you are under a lot of stress you are likely to be emotionally sensitive. Stress causes people to deviate from their routines (like going to the gym after work) and causes them to adopt a "who cares" attitude. Typically stress will also causes you to dwell in negative thought processes. Instead of having a positive outlook on life, you find yourself expressing a negative demeanor in most, if not all situations. Stress has also been linked in increases in diseases and health ailments such as heart attacks, depression and hypertension to name just a few. Ultimately you want to decrease you stress levels as much as possible. Doing so will have a profound effect in all aspects of your life!