When Lying In Bed, I'm Fine As Long As I

When lying in bed, I'm fine as long as I lay flat on my back. But if I turn to either side, I suddenly become dizzy and see the room spinning. If I try to get up, I can't stand as I get disoriented. Another time the dizziness occurs is when I'm 'making' the bed after getting up. When I reach across the far end of the bed to straighten the bedspread, I suddenly become dizzy and have to lie down. Strangely enough, when I'm up and about and even while driving, the dizziness does not occur.