What Causes A Hard Lump And Severe Pain Below The Knee?

I fell about a week and a half ago. I hit the outside of my left leg just below my knee on the way down. It was painful and I could hardly walk on it the rest of the day/next day but it never swelled or bruised. Now, it still hurts (especially when I bend my knee). I noticed this morning, I have a hard lump there. I have wrapped it in coban during the day, which helps a little. At what point should I be concerned?