It happens to me all the time when I take adderall.
I believe it has to do with a rapid change in your blood pressure, and most likely occurs when you go from a sitting or laying down position quickly to a standing up position.
It seems to happen to a lot of people, and I don't think that it is something to worry about, however, if it's happening to the point where you are blacking out and falling then you should see your Dr.
Sometimes I stand up and your vision goes black for a second, that's normal. Standing up and literally blacking out, and then falling to the floor or something, that's what I mean, and that's what I believe you should see a Dr about.