What could cause my left leg to go completely numb if I lie on my back?

PROBABLE SCIATICA. Your symptoms are probably due to a 'compressed' lumbar nerve in your back (likely l3-4). A lumbar MRI scan may confirm the diagnosis. You should get seen for proper treatment which may include an initial short course of oral steroids (e.g. Medrol (methylprednisolone) pack). Warning signs for a serious condition, spinal stenosis (pinched spinal cord), are : leg weakness and/or bladder/bowel changes. Hope ur better!