Next time when you decide to stand up do it slowly not in a sudden motion. Sometimes the blood in the brain goes down with gravity when we suddenly stand up after being seated for a while. But like special mom said this could also be low blood pressure resulting from low food intake . So, make sure you keep a check on that. Other than that are there any medications you are already taking? maintain a balanced diet, good sleep and a few days of exercise during the week and you will be feeling better within no time. Take care
This sounds like low blood sugar. Do you eat regular meals? Snacks? Even something little will keep your sugar up. AT 16, you need to tell your mom. good luck