Yes breast feeding is associated with consumption of more calories in milk production. But it does not result in rapid weight loss. The cause or reasons for being underweight are chronic diseases, very rapid metabolic turnover, very high stress levels, hyperthyroidism, hyperactivity (Attention Deficit Hyperactivity Disorder - ADHD), depression, and lack of physical activity. You may be underweight due to hereditary reasons.Meet your family doctor to rule out any medical reason for weight loss. In gaining weight you should burn less calories than you are taking. so here are some good tips for gaining weight. Eat six smaller meals a day. You should also eat more energy dense foods like healthy oils, dry fruits, nuts, pulses, etc. along with some starchy foods like potatoes, cereals, bread, pasta, rice, etc. Mangoes are rich in calories and carbohydrates; include them in your diet.Try to Eat healthy food frequently, 5-6 times a day - 3 larger meals and 3 smaller meals alternately. Separate them by at least 3 hours so you can digest each meal fully. Don t go without food for 5-6 hours after having a major meal.Exercise regularly for healthy weight gain.Exercise keeps the metabolism high. Eat adequate protein for muscle mass.You should daily eat about 2.2g of protein per kilogram (1g per pound) of your weight. Thanks