Why do I get dizzy when I lie down?

Problem in inner ear. The most likely cause is something affecting the nerves or the small bones in the inner ear. Could be a virus (acute layrinthitis) a bacterial infection (sinus, ears, nasal passages), 'benign positional vertigo' adverse medication affect. Aok if rx or goes away within 2wks. . But if goes on longer, very severe, need further tests, like a brain scan (including internal auditory canals).