I've been undergoing testing for years for similar symptoms, along with a ton more.
The first things all my doctors suggested was depression. This is not the case for me, but believe me, they will suggest it, and it's something to look into.
I was finally diagnosed with fibromyalgia and chronic fatigue syndrome, but my doctors are still testing so I'm not sure how confident they are in their diagnosis.
Just a few things that were brought up as possible causes for fatigue, naseau, foggy thinking, achiness, etc, etc, were a thyroid disorder, Lyme disease, B12 deficiency, anemia...
There are so many things that could be the cause of what you're feeling. I would defiitely agree with the suggestion to go to a doctor. It might help to keep a journal of how you feel every day for a while, and write down all your symptoms, etc. I didn't do this at first and I would constantly forget to tell my doctor about things.
Good luck!