I have had this pain before, and was about a week or two after I slipped and fell right on my bum! I bruised my tailbone. If it does so bad you can't even stand up or walk, that probably means you shattered it. It is just bruised most likely
Did you injure yourself? It's not hard to bruise your tailbone. If so, the pain will diminish over the next few days. Also, it is not uncommon to develop a pyloral cyst in that area. If its not bruised and/or the pain persists, see your doctor.