My child passed out while getting blood drawn why? and she threw up also and her body was shaking and she sound like she was choking

Vasovagal syncope: Vasovagal syncope is the most common cause of fainting and occurs when your body overreacts to triggers, such as the sight of blood or extreme emotional distress. The trigger results in vasovagal syncope, a brief loss of consciousness caused by a sudden drop in your heart rate and blood pressure, which reduces blood flow to your brain. The vomiting and shaking were most likely caused by this also.