Lowering blood flow to a girl's brain can make her faint, according to NHS Choices. Fainting is caused by an automatic nervous system malfunction or by low blood pressure.
The automatic nervous system malfunction can be triggered by overheating, by viewing a traumatic sight, and by experiencing intense pain, notes NHS Choices. Less extreme causes may be coughing or sneezing, which trigger the body's response to cut blood flow to the brain. Standing up very quickly reduces blood pressure and blood flow to brain, potentially causing fainting. Dehydration and ingesting certain medications can also reduce blood pressure, potentially causing a person to faint.