My 12 day old baby just projectile vommitted. It definitely wasn't spit up, it was vomit. What could be the cause of this?

Eating too much. Babies will vomit if they eat too much, as their stomachs fill, and then their esophagus, and then they vomit. If you are feeding the child more than 2 oz at a feeding decrease it to that level for a few days, then try increasing. If not better, have the child seen.
2 basic issues. The more common cause would be progressive gastroesophageal reflux which is a form of "spitting up".The other would be an excessive tightening of the muscle that holds food in the stomach before releasing it into the intestines.Pyloric stenosis or spasm can produce rather dramatic vomiting.It can be diagnosed by exam or tests.