I'll get dizzy (though I haven't since pregnant) when my blood sugar is low, or if I haven't been drinking enough water so my blood volume is low.
I have very annoying blood sugar issues, I even tested it when I worked at a group home and it was 32 one day, 37 the next day and normal is 76-125! So especially since a baby is depending on my blood I make sure to eat regularly, eat enough protein and fat (takes a long time to digest and stabliizes blood sugar) and don't have much caffeine, which lowers blood sugar.
If I don't drink enough water than my blood volume is really low, which causes low blood pressure, so when you sit and then stand there isn't enough blood to pump it up to your brain, which will cause dizziness. From what I understand, it's your body's last-ditch effort to try and get you to lay down so that the blood can get to your brain.
Oh, and I'll also do the same sort of thing if I'm really tired, but that's because of lack of sleep. My body is nit-picky and likes everythign to be right, I guess

Cara