sore feet when standing

My feet are fine first thing in the morning, but walking and particularly standing can only be tolerated for a couple of minutes before they begin to ache. After a long day it can get so bad that even resting them on the ground while sitting is painful, and the only thing that helps is to elevate them in midair. This has been going on for about a year now and was initially diagnosed as plantar fasciitis although my podiatrist now agrees I don’t have that. He says he doesn’t know what the problem is, that I have “tired feet”. This condition is really disabling – I can barely shop for groceries since it is so painful to wait in line. I have tried everything suggested – stretching, the “boot” at night, cortisone injection, orthotics, massive quantities of ibuprofen, icing, resting, and even accupuncture, and nothing seems to make any difference. I am a 47 year old woman in good health otherwise although I had breast cancer in ’01 and currently take Tamoxifen. Any ideas? Thanks so much.