So far, your answers indicate that neither something you take nor any recent illness is contributing to your lightheadedness with standing.
It may be that you just don't drink enough fluid even though you may not be thirsty. If your urine has a darker yellow color than usual, this would support mild dehydration.
Increase your fluid intake. If you don't have high blood pressure, fluid retention, or heart failure, consider adding a little salt to your diet.
Please continue with the guide.