POTS is a form of dysautonomia which is a disfunction in the autonomic nervous system. It can cause lightheadedness, heart rate spikes and fainting

VenusPoolingIs Venous Pooling Causing My Tachycardia? The common assumption is that venous pooling (blood pooling in the legs or abdomen) is the primary cause of symptoms in Postural Orthostatic Tachycardia Syndrome (POTS) patients. However, some POTS patients feel worse when raising their arms above their head, which should theoretically improve blood return to the heart if venous pooling were the sole issue. The problem may instead be related to constricted blood flow or compression somewhere

What is POTS and Dysautononia by Dr. David Saperstein