It's called "orthostatic hypotension" or "postural hypotension".
I've had it my whole life because I have naturally low blood pressure. If it's not causing you to pass out and your doctor has determined there is nothing underlying that is causing it then it's not anything to stress about. (Ha, easier said than done, right?)
I always make sure to get up slowly if I've been sitting for a long time. I sit back down for a second if I get tunnel vision or dizzy, then slowly get back up again. It's worse when I'm tired, dehydrated, not eating well, etc.