What Causes A Painful Lump On The Back Of Thigh?

For about the past 2 years I have had a lump on the back of my right thigh. It never really hurt or changed. So I personally didn't think much of it. However this past week it has grown a lot bigger, now it is really discolored and is extremely painful to where it brings tears to my eyes when I sit down or when I touch around the lump ect. Any advice of what this possibly could be?