puking

My baby is 4 weeks old as of Thursday... the whole month he's been alive he's maybe spit up twice..between yesterday and today he had projectile vomited 3 times. He is drinking breast milk as well, not formula. Also since yesterday he woke up and kept gasping for air like he couldn't catch his breathe. I brought him to the dr and they told me his lungs sounded fine and oxygen levels were 99%. What's going on with my little guy?