Dehydration can be a cause. When you become dehydrated, your body loses more water than it takes in. Even mild dehydration can cause weakness, dizziness and fatigue.
Or it can be you have naturally low blood pressure (normally thought of as being lower than 90/60 mmHg) is unlikely to mean there is anything wrong with you. It may be normal for you and in fact is beneficial in many ways, as it can reduce your chance of getting diseases such as coronary heart disease and stroke.
Low blood pressure is only considered to be a problem when it causes symptoms such as fainting. This usually only happens if there is some underlying problem which means your blood pressure is not being controlled properly.