why would blood pressure drop

Low blood pressure, also called hypotension, is blood pressure low enough that the flow of blood to the organs of the body is inadequate and symptoms and/or signs of low blood flow develop shock.
Low pressure alone, without symptoms or signs, usually is not unhealthy

The symptoms of low blood pressure include lightheadedness, dizziness, and fainting. These symptoms are most prominent when individuals go from the lying or sitting position to the standing position (orthostatic hypotension).

Common causes of low blood pressure include a reduced volume of blood, heart disease, and medications

The cause of low blood pressure can be determined with blood tests, radiologic studies, and cardiac testing to look for heart failure and arrhythmias.
Treatment of low blood pressure is determined by the cause of the low pressure