In your case, it's clear that your blood pressure level is too low when you feel weak and dizzy. When this symptom occurs, you should immediately lie down. If that is not possible, at least sit down. If you continue to stand while feeling lightheaded, you could faint and hurt yourself.
There are many possible causes of low blood pressure. My approach is to review all medications that the person is taking. If you take blood pressure pills or heart medications, the dose may be too high for you. Many other medications can lower blood pressure.
Next, I make sure the person has been drinking enough fluids. You might not feel thirsty, but could be mildly dehydrated. Also, many people restrict their salt intake. Usually this is healthy. But people with low blood pressure may actually need to increase the amount of salt in their diet.
These two factors, medications and dehydration, account for the great majority of low blood pressure episodes. If neither of these applies to you, you could have postural hypotension. In this condition, blood pressure falls dramatically when someone stands. Usually this is caused by problems with the nerves that help regulate blood pressure. People who have had diabetes for a long time can have this problem.
You should definitely contact your doctor if you think one of your medications is causing the low blood pressure or the problem does not quickly go away by increasing your fluid intake.
Find out more about this book:Harvard Medical School Hypertension: Controlling the 'silent killer'