Sudden weight loss or gradual weight loss may be due to different causes. I assume your weight loss is not abrupt enough to warrant an immediate medical attention, it is a gradual loss.
Anyway gradual loss of weight again can be due to a number of reasons. First and foremost foodstuffs and diet plan versus your workload ( calorie loss ). If you take less calorie and do workouts heavily or do lots of physically demanding activities that will cause you your weight. Enrich your diet according to your calorie demand. Ensure every portion of diet carbohydrate, protein, fat ( fat is necessary, don't forget that ), fibre, minerals and vitamins. Fat is one of the major source of energy from food. Don't ignore that point. Use olive oil, safflower oil, sunflower oil for good cholesterol. Take enough of carbohydrate and dietary fibers also add enough protein ( ideally egg protein ), as protein