According to Healthline, dehydration is one common cause of a sudden drop in blood pressure. Dehydration occurs when the body does not have enough fluid and can cause your blood volume to drop, leading to a decrease in blood pressure, per Mayo Clinic. This can happen due to factors such as not drinking enough water, sweating heavily, or having diarrhea or vomiting (via Mayo Clinic).
Another common cause of sudden drops in blood pressure is low blood sugar, also known as hypoglycemia (via the Mayo Clinic). This can occur in people with diabetes who have taken too much insulin, missed a meal, or exercised more than usual.
In some cases, sudden drops in blood pressure can also be a symptom of a more serious underlying condition, such as septicemia, anemia, or a heart attack, says the clinic. If you experience symptoms of hypotension, it's important to seek medical attention to determine the underlying cause and appropriate treatment.