you should go to your doctor to get bp checked it seems very low from what you're saying, its a sign its too low if we go dizzy when we get up, it really needs to be sorted, you'll probably be told you're fine unless you pass out
it sounds like a blood pressure issue to me,go to your doctor to get it checked out
For the standing up part it sounds like you Blood pressure is dropping momentarly so the oxegen cab reach your brain and that's what causes the light headed feeling but I don't know about the sitting down!
I am no doctor but can relate because I have been feeling light headed alot for over a year now. Though for me it went away fir a few mounths. And I think today I may have actualy fainted. So I know have had 3 big light headed spells. The first time was last summer July 22nd my legs shook. The second time was the middel of the school year I fell at the top of the stairs though I think that was my body self conshesly so I dident fall down the stairs and third time today. All times I have no controle over my body for 30 seconds or more but for the first 2 times was completly awasre if everyhing goin on. Today was kinda a blur! Does any if this sound familiar???