Why is my newborn losing weight?

ALL babies do. This is a normal process in newborns. They body is made up with more water than older babies and this is mostly water weight loss. Normal babies lose up to 10% of their birth weight in the first few days, then get back to birth weight by 7-10 days of life, and then gain about an ounce per day or a pound every two weeks. If you are in those ranges, then you are alright.
May be normal. It is normal for newborns to lose up to 10% of their birth weight within the first week of life. This weight loss represents excess body fluid. By two weeks of age, your newborn should be back at his birth weight, if not above. If your newborn's weight loss persists or seems excessive, start keeping track of his daily wet and dirty diapers and see your pediatrician.