I have a fainting disorder and low blood pressure. That literally just happened to me today. I was sitting and was very dizzy. However, I don't really know why it actually happens. I know this doesn't help, but you're not alone. Lol. It happens.

I have chronic nausea so it hits me whenever, however it wants to. I wear nausea bracelets instead of adding to my medication list. They're stretchy wrist bands with little white balls on the inside that sit on the nerve in your wrist that help relieve nausea when pressed. I get mine at CVS

This happens to me all the time. No clue what it is but you're not alone

I have POTS (Postural Orthostatic Tachycardia Syndrome), which does this when I change positions. I also think there is something else going on, as I have chronic hypoxia and hypoxemia with no (yet) known cause, but I'm guessing it is my lupus.