3 days ago, I fell down 12 stairs on my rear end. Massive bruising and swelling to right buttock, pain in spine, do I see a doc or just ice and rest?

Never a bad idea. To see your doc in order to monitor for resolution of the symptoms and get a sense of the damage at baseline. In case persistent, worsening or new symptoms it will be much easier to have them addressed promptly. In the interim rest and ice +/- inflammatories as needed sounds like the best you can do.