Babies all spit up. As long as it's not a full feeding and only like a teaspoon or two at a time its fine. Although babies spend 50 weeks maturing inside of us their digestive system is still very immature at birth. It takes about 4-6 weeks to fully mature and develop all the needed good bacteria in the gut.
Not burped good enough but also my doctor told me that baby may be eating to much.
Burp the baby more often during feedings. Some babies just spit up more. My son spit up everytime. Reflux may be an problem too.
Maybe the baby needs to be burped more.