Why do i get so light headed and nauseous when i laugh?

Laughing: Laughing increases intra-abdominal pressure, which also compresses a large vein called the inferior vena cava, which decreases blood flow to the heart and can potentially affect cardiac output by decreasing filling of the heart. But, you also need to evaluate for other causes which include anemia, infection, dehydatrion, cardiac and neurological problems.