What Causes Lump And Bruising On The Head?

My mom, age 78, fell and bumped her head on the sidewalk a few days ago. No change in behavior, no head aches, just a knot and a lot of bruising. It's been about 5 days ago and she says she feels fine, is doing everything she does normally, but she is concerned about the knot remaining. What kind of concerns should we have?