What Causes A Soft Tissue Lump On The Side Of Knee?

What is a soft tissue lump? I had an xray today on the side of my knee. I previously had an infection in the lower part of my leg after cutting it on the dishwasher. I was on antibiotics for a few days but now have a small lump on the side of my knee. It does not hurt. My doctor took x-rays today but now wants an ultra sound.