Thanks for posting your query.
is caused by long standing because it may lower your blood pressure. It can be triggered by anti-hypertensive drugs you have been taking.
Sometimes it can be due to dehydration
, as you have low iron levels you must be checked for anemia.
Take plenty of water, avoid long standing. It is better to take anti-hypertensive drugs before going to bed. You may talk to your doctor if he thinks about changing the dosage of drugs.
Hope it helps.