I fell on some icy steps 2 and a half weeks ago and had a bad bruise and swelling, now the bruise and swelling are almost all gone but now my lower back and tailbone hurt really bad.  I have noticed that I have a bump at the top center of my bottom and it is very painful do you know what that could be?
It seems that you have injured your coccyx which forms the tip of the sacrum of hip. Although the soft tissue swelling has subsided, the bump that remains is due to a bony injury as mentioned. You need to get an x-ray done and consult a doctor with the report. You need to use a donut shaped cushion to sit on. That will give you some relief. You may also use hot compress.