3 weeks old, throwing up and not pooping, what could be wrong?

3 week old vomiting: This is concerning - if this is really vomiting and not just baby spit-up, it can be a big problem for a three week old. It could be a formula intolerance, but then usually they have diarrheal stools. I would also be concerned at this age about a condition called pyloric stenosis, where the stomach cannot empty correctly. Baby needs evaluation in next few days if not sooner!