There are a couple vein compression syndromes that can cause your symptoms. One is Nutcracker Syndrome (Left Renal Vein Compression) and is a known cause of Chronic Fatigue, which it sounds like you may have. It also causes abdominal flank pain, usually left sided. It can also cause compression of the splanchnic vein, which could cause nausea and vomiting.
Another one that I have is May-Thurner Syndrome, which causes many of the same symptoms of Nutcracker Syndrome. An Interventional Radiologist or Vascular Surgeon would be good doctors to get checked for these conditions. Just an idea as I know most doctors don't know very much about these conditions and wouldn't think of them.
I know someone that has Nutcracker and May-Thurner Syndromes and has a support group forum for these undiagnosed symptoms. If your interested in more info, send me a private message and I'll be glad to help!