My arm has been sore from shoveling snow. I want to know how I made it hurt so much.

For pity's sake, millions of us have sore arms from shoveling snow. You can strain any number of joints, tendons, and muscles from your trapezius (large back muscle) to your fingers. If you can't even twitch without being racked with pain, I suspect you pinched a nerve. The treatment is soaking in the hottest water you can stand, or standing in the shower with the water beating down on your arm, or a heating pad with a damp washcloth against your arm. Don't slouch on the couch or in bed in one position for very long, and keep moving your arm gently. Take Tylenol and ibuprofin. I alternate between the two every 4 hours as directed. If pain lasts longer than a week, see a chiropractor, or a clinic that does physical therapy.