I dunno much 'bout Lucid Dreams, seeing as I'm a newbie. Though, the getting up and falling down thing is common. The same thing happens to me. If you see stars while getting up and feel light headed, it's just a rapid change in your blood pressure. Try getting up slowly, it should be better. Though, getting up quickly isn't dangerous, just a bit inconvenient.
(I wikipedia things a lot. This particular thing, phosphenes. "Seeing Stars.")
http://en.wikipedia.org/wiki/Phosphenes
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