This unusual syndrome causes your heart to race when you stand up

A rapid heart rate—tachycardia—is a key feature of postural orthostatic tachycardia syndrome, or POTS, a mysterious condition that involves the nervous system and can cause a constellation of symptoms, including fatigue, shortness of breath, headaches, and brain fog, as well as pain in the chest, extremities, or elsewhere in the body. At least half of all cases of POTS are thought to be triggered by an infection (such as mononucleosis), which then spurs inflammation or an autoimmune response. Treatment includes drinking lots of water, eating lots of salt, and graded exercise training.