What Causes Pins And Needles In Arms When Bent Across The Body?

When I'm in bed at night I sleep best on my side with my arms bent across my body. I've been unable to do this lately as my arms (specifically the left one) ache and become dead causing pins and needles when I straighten them. I have to try and sleep now with my arms straight which isn't comfortable. Do you have any idea what would cause this?