Hi Nez,
Your answers to my prior questions lead me to believe that all 3 factors are playing a role in your headaches (although they are likely not the only factors).
From where you sit, if you can reach out and physically touch the computer screen with your hand, then you're too close to it. The high 'brightness' of the computer screen is almost certainly causing you to blink less often, which keeps your eyes open for longer periods of time and is a causal factor in headaches. The slouching also impacts a variety of different things - whether you realize it or not.
If you're working on a laptop computer, try placing it on a stack of books, so that the screen is at eye-level (and so you're not looking 'down' at it). Make sure to place the computer more than an arm's length away and sit up in your chair. Get in the habit of taking lots of notes while you answer GMAT questions - that way, you'll have to turn away from the bright screen (and you won't end up staring at the screen non-stop).
GMAT assassins aren't born, they're made,
Rich