Low Blood Pressure

CAUSES

• Pregnancy. Because a woman's circulatory system expands rapidly during pregnancy, blood pressure is likely to drop. This is normal, and blood pressure usually returns to your pre-pregnancy level after you've given birth.
• Heart problems. Some Heart conditions that can lead to low blood pressure include extremely low heart rate (bradycardia), heart valve problems, heart attack and heart failure.
• These conditions may cause low blood pressure because they prevent your body from being able to circulate enough blood.
• Endocrine problems. Thyroid Conditions — such as parathyroid disease — adrenal insufficiency (addison's disease), low blood sugar (hypoglycemia) and, in some cases, diabetes can trigger low blood pressure. 
• Dehydration. When You become dehydrated, your body loses more water than it takes in. Even mild dehydration can cause weakness, dizziness and fatigue. Fever, vomiting, severe diarrhea, overuse of diuretics and strenuous exercise can all lead to dehydration.
• Far more serious is hypovolemic shock, a life-threatening complication of dehydration. It occurs when low blood volume causes a sudden drop in blood pressure and a reduction in the amount of oxygen reaching your tissues. If untreated, severe hypovolemic shock can cause death within a few minutes or hours.
• Blood loss. Losing a lot of blood from a major injury or internal bleeding reduces the amount of blood in your body, leading to a severe drop in blood pressure.
• Severe infection (septicemia). Septicemia can happen when an infection in the body enters the bloodstream. This condition can lead to a life-threatening drop in blood pressure called septic shock.
• Severe allergic reaction (anaphylaxis). Anaphylaxis is a severe and potentially life-threatening allergic reaction. Common triggers of anaphylaxis include foods, certain medications, insect venoms and latex. Anaphylaxis can cause breathing problems, hives, itching, a swollen throat and a drop in blood pressure. 
• Lack of nutrients in your diet. A lack of the vitamins b-12 and folate can cause a condition in which your body doesn't produce enough red blood cells (anemia), causing low blood pressure.