3-week old vomits after eating once per day?

Our 3-week old son is projectile vomiting large amounts of food (both milk and formula) about once per day. It is not odd colored or anything like that, nor is it after every feeding. Our doctor said not to be concerned unless it is after every feeding, or if it is green and other colors. However, this can't be normal. In addition to vomiting, he also coughs a lot while eating and sometimes after eating. We use Dr. Brown's with Level 1 as well as Born Free bottles with Level 1 . He was born a month early and is only around 6-7 lbs in case that matters. Any ideas? Could it be reflux? Should we be more insistent with our doctor?