But most probably low blood pressure, whether systemic, or caused by antihypertensive medication or booze. Are you on either? This may be technically labile hypertension or postural hypotension, and may make you faint clean away.
Before I frighten you, the fact that you say "when I stand up" probably means it's not serious whatever is causing it, but if it comes on for no apparent reason, it could be a bit of plaque or arterial spasm or something causing a TIA (transient ischaemic attack), tho this is likely to be accompanied by other quite frightening symptoms. If it is not, it may be amaurosis fugax (fleeting blindness), which is still quite serious. (Mine was not as fugax as I would have wished.)
So if you havent mentioned it to your GP, who will have given you a bowdlerized version of what Ive given you, and if you're not on antihypertensives and/or hitting the bottle, better do so before too long!