What Causes Dizziness When I Stand Up?

Hi. Usually, I get a little lightheaded when I stand up too fast, I just hold onto a wall for a couple seconds and it goes away. My vision will go dark/blur and then it will all normal. Yesterday I was got up off the couch to go to the kitchen after aprox 30min sitting, After standing i felt the light headedness coming so i held the wall as usual but I had, what I can only describe as a mini seizure both arms violently jerking around uncontrollably, this lasted for about 30sec and then about 5sec later my vision returned, it was still a further 5min after sitting down that I started to feel fine. Any help greatly appreciated Aubrey.