What Causes Pain In Lower Back And Buttock Area?

I have fallen twice in the past 2 weeks. Once was on slick pavement and 3 days ago I fell down 2 stairs. After the first I noticed minor pain in my lower back and right buttock area. After the second one I have more pain in my buttock area and right groin area. Sometimes the pain seems to go to my foot. It feels much better when I stand. Sitting is uncomfortable.