What can be done about extremely swollen legs and feet?

I used to get swelling on my left leg and foot. I recently had a knee replacement on my left leg and now my right leg as well as my left leg are 3x their usual size. I am a diabetic with high blood pressure and have had to have a thyroid removed. What could be causing this swelling? It is very painful and I can barely walk. The bottom of my left foot is turning black. I have had my circulation tested and they claim it is fine and that I do not have any heart issues. They checked the left knee for blood clots and say there are none there but did not check anywhere else.