What Causes Back Pain After Lifting Heavy Weight?

I was carrying a relatively heavy vacuum cleaner up the stairs last night when my back "went out" on me. I was basically left motionless on my back in bed thereafter and almost any movement causes a sharp pain on the right side of my spine directly in the middle of my back. Is this likely muscular or could it be a disc thing? I can't even get in a car to go to a doctor I'm in so much pain now 16 hours later...