Fainting happens when your brain doesn’t get enough blood, and that causes you to briefly lose consciousness. What Causes Fainting: Hyperventilation Low blood sugar. Being pregnant Having anemia. Emotional stress. How to Diagnose VASOVAGAL SYNCOPE How to Tell If Someone May Faint: *The patient says they feel nauseous. * Yawning frequently * Feelings of restless * Complaints of feeling lightheaded or dizzy * The skin looks pale, feels cool and may be clammy or sweaty * Complaining of tightness in the chest * Having palpitations