It sounds vascular to me as well. When we stand all our muscles especially those in the lower leg constantly contract and relax as our natural sway occurs to keep us balanced. These contractions can help facilitate the movement of blood, thus helping pump blood into the area and byproducts out. Could be why you feel better standing. Do you mind me asking how old you are and what your weight is? Also any other medical history? Do cuts/blisters heal slowly on your feet? These also can be signs of vascular problems? Definitely get looked at.