Could be a "recession". Along with the national economy, apparently my mouth is experiencing a recession as well right now. Or so says the dentist from my checkup 2 days ago.
The gum recedes from the tooth and leaves the root exposed. The root has no enamel, so the nerves are directly exposed to contact. My dentist says cold and contact with metal typically causes the most sensitivity. I could feel them poking it with the metal instrument while they were scaling the teeth clean. Apparently it doesn't cause medical issues or further dental issues, it just causes pain and/or sensitivity. Dentist recommended sensodyne toothpaste and said the recession is unlikely to grow back to cover the root.