This evening I woke up from a nap and felt incredibly nauseous. Sometimes when I'm really nauseous, I start to faint. So even though I was sitting in bed, I suddenly got all sweaty, started to lose my vision, got really dizzy, etc. The ironic thing is that feeling faint makes me more nauseous, which makes me feel more faint...you get the idea.
Fortunately the worst of these episodes doesn't usually last long.