What Causes Legs Clotting Feeling Behind The Knee?

Hi, Im a 30 year old female and about 6 years I underwent a Phlebectomy because of aesthetics reasons. Overall, I am healthy persons, social smoker, no cholesterol, or hormone imbalances, healthy heart, as I said, a healthy person. However, I do feel tired sometimes at the end of the day and from time to time I will get a blood clot feeling on the back of my legs behind the back part of my knee. Is it something I should worry about? Could it be a blood clot? What kind of preventive measures can I do? Please advice.