I Just Suddenly Developed A Large Lump On The Inside

I just suddenly developed a large lump on the inside of my left thigh mid way between my knee and groin. I had arthroscopic surgery on L knee a month ago and have been attending therapy. The leg has remained swollen down to and including my foot as well as remaining numb on the shin area. Any ideas? Im really concerned about this lump which literally just appeared. I have been sitting watching TV with my leg up on a footstool.