The most likely cause of your dizziness when changing positions is orthostatic hypotension. Orthostatic hypotension occurs when a person rises to a standing position from a lying or sitting position and is a result of falling blood pressure of 20 mm Hg systolic or 10 mm Hg diastolic with or without an increase in pulse rate. The symptoms typically include dizziness, light-headedness, temporary blindness, body dissociation, extremity tingling, or even fainting. These symptoms are a result of insufficient blood flow to the brain. Common causes of orthostatic hypotension include volume depletion, neurologic abnormalities, and medication.
If raising your arms causes you dizziness, you should be evaluated by a doctor for a physical exam – especially if you can consistently replicate the symptoms. Commonly, compression of the large blood vessels in the neck/upper thorax can produce a dizzy sensation. However, rarely, tumors of the upper thorax can produce numbness/weakness of a limb as well as dizziness upon raising the arms.