Vascular or nerves. You should talk to your doctor about this. This could be due to blood flow or nerves, or some of both. There could be a problem getting blood to your feet, back from your feet, or compression of nerves (which can also be because of blood flow). Lastly, this could all be caused by muscles that impinge on your vessels or nerves. A detailed history of your symptoms should clear things up.
Numbness. Do really they get numb or fatigued? If numbness is what you feel, there are possible more nerve ending problem. If they are fatigued, could be due to deconitioning, nerve or circulatory problem. Needs evaluation.