| I agree with J 9 that postural blood pressure, defintely appears to be a significant factor. The headache does seem to me to be vascular in nature. It's hard to rule out that it isn't a migraine, albeit very severe.
Any other information you could give us -- diet, meds, supplements, etc. -- would be helpful, but you could try boosting your potassium level by getting more in your diet. Please don't take potassium supplements though, unless under medical supervision. Low blood sugar could as well cause headaches such as you describe, though you would likely experience some ringing in your ears at the very least.
If the headache becomes unbearable, or is accompanied by symptoms such as serious visual disturbance -- more than what is common with a typical migraine -- it is always best to seek out emergency medical help. Though more serious causes or complications are relatively rare in people that present as otherwise healthy. Location of the headache can be something of an indication of it's type, though this isn't always reliable. | 
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