Leg below knee including feet swells up and feels very heavy and sometimes numbness too, after sitting in chair for 10-20 minutes.?

Blood flow issue: Veins are vessels that bring blood from the feet back to the heart. Veins use valves and the muscles of the leg to aid in moving blood up the body. When veins aren't working properly, legs can swell. Compression stockings may aid in preventing swelling. A phlebologist may be able to repair problem veins and prevent it from returning.

Venous insufficiency: Since veins do not pulsate, they rely on muscle to contracted and in turn put pressure on veins to send the blood back up to the heart. When seated for 20 minutes your leg does not have a chance to pump the blood back up, it swells, applies pressure on the nerves, and then you get the numbness. I would try to elevate my legs while sitting or try to move as much as possible. See a vascular doctor.