What Causes Dizziness While Lying Down?

Last night I woke an felt dizzy. Thought how do you get dizzy when you re asleep? Then this morning (it was light out) I rolled over to my right side and got the same sensation. So I opened my eyes and the room (or my eyes) were spinning out of control for several seconds. I rolled to my left side & it stopped & I fell back asleep. I woke about an hour later, turned to my right side & the same thing happened so I closed my eyes & still felt very dizzy. I also have a slight headache at my left temple. It just happened again when I bent down to the right under my desk & I thought I d better look this up & find out whats up?