What could make my six month old to be vomiting?

Vomiting: The list of causes of vomiting goes on and on. But the most common reasons at this age are a: reflux, and b: gastroenteritis (most likely viral). In both cases you have to be mindful of your child's hydration status. But differentiating between the 2, or any other possible causes, is important as the treatment for each condition is different. It's best to consult your doctor to help find the cause.