Whether you have blood clots or not, May-Thurner Syndrome should be ruled out as a cause of swelling in the legs. Even without blood clots, this condition can cause congestion of other veins around it and swelling in the thighs and legs, especially left-sided.
I have the same symptoms well over 2 years now, my doctor just says it's the veins . I'm due to have one left leg stripped end of May 2017. Hopefully I may see an improvement.