Fell on back at about 10mph and am having a lot of pain. What should I do?

Go to ER. If you fell on your back that hard and are having that much pain - you may have a fracture. Your should go to an urgent care center or er and be properly evaluated.
Fall on back &pain. You should see your doctor asap and get evaluated and treated properly, you may need pain killers, muscle relaxants with or without physical therapy and imaging studies if indicated.