when i sit or stand i feel dizzy?!

Tight neck muscles are the cause of your problems. When the neck muscles tighten up they restrict everything going in and out of the head, including the blood. When there is a reduced blood flow to the brain you will have a lowered oxygen level there as well. You will know when the oxygen level has dropped when you feel very tired. If the level drops more you will get dizzy, light headed. And more you will pass out. The nerves for your eyes go to the back of your head, passing next to the neck muscles, as they go to the top of your head. When the neck muscles are tight they can press into the nerves for your eyes which gives you pains on the back of the eyes where the nerves is attached. When you hold your head still as you would on the computer your neck muscles would tighten up more to press into those muscles to cause the problem. To get rid of your problems you have to free up your neck muscles and here's how to do that:
Neck
Put your hands alongside your head so your thumbs are on the front of the muscle under your ear and your fingers are on the back of the muscle behind your neck. Squeeze your thumb and fingers together and hold. Relax your body. When your fingers and thumb touch, about one minute, slowly lower your head as far as you can, release the pressure but hold your neck lowered for another 30 seconds.
For best results relax your body first by taking a deep breath and exhaling then remain this relaxed.

Have you drank any water in the last couple of days? If you haven't, DRINK WATER. But if you have drank water and you are feeling dizzy, you might want to consult a doctor. Feeling dizzy is not good.