What Causes Swelling In The Left Foot?

Hi, I m a 120 lb. 63 year old female with no diabetes, arthritis, or heart problems. For the past couple of months the top of my left foot and ankle (and somewhat up my leg) has been swollen. It has felt tight with a pulling sensation with occasional shooting pains. I have been to the Dr. and an ultrasound on my leg has ruled out a blood clot. I m not on any medications except I have been put on Lasix which doesn t seem to help. I ve also put myself on a low salt diet. I have never had an injury to this foot. My blood pressure is very good. I ve been wearing compression stockings which only seems to minimally help. I do have a barely visible varicose vein on the inside of my calf that I ve had since my 20 s. My Dr. thinks I may need to see a vascular surgeon. In the meantime, I m searching for possible answers. Thank You!