Both physical and mental tensions of stress consume a lot of energy and have a draining effect on the body. This draining effect, if it is ongoing, may manifest as a weight loss.
Stress is a form of response to a threat, and the body tries to be prepared and ready. This need for immediate readiness may put a block the desire to eat, as a person with a full stomach is not as responsive to an immediate threat.
Of course, the stress we generally experience is the result of a sort of lingering, long term threat, but the body doesn't know that and reacts to it as it were immediate. So, it not uncommon that a stressed person feels a discomfort in the stomach area and experiences loss of appetite.
If there are emotional issues, they further induce the body to try to feel clear, clean and transparent, and these again influence the digestive processes to be put on hold. Many people people take up smoking because of stress; this gives a false sense of fullness and reduces appetite even more.
Unless the stress is resolved outright, stress related weight loss is not very easy to counter. Don't smoke, take the time to choose healthy, wholesome foods and also take the time to relax while eating. Avoid junk food and consume food that is easier to digest. Jumping right back into stress does not help with digestion - take things slowly and try to find a natural rhythm in life.