What Causes A Painful Lump On The Inner Side Of The Leg?

I have a mass on the inside of my left leg. I am a white 45 year old woman who is very over weight. When this mass first appeared, I assumed it was fat and was too embarrassed to ask anyone about it. As time has gone by, the mass has gotten larger and painful. I have times when I can barely pick up my leg and walk nod then days will go by and I will be able to walk. If I walk too long the front part of my left leg will go numb to my knee. I have had a ruptured disc in my back about ten years ago and recently had rods put in my back for a collapsed disc area. (L5/S-1) This is definitely a different pain. This pain feels like it goes from the mass up to my groin and into my pelvic area. I have extreme pain when I try to cross my leg or pick it up off the bed. I have had a complete hysterectomy. I am concerned that this mass could be cancer. What should I do?