I've been experiencing dizziness, fatigue and little to no appetite for going on 8 weeks now. What could possibly be ailing me?

And before someone jumps on me for asking this on an internet forum instead of to a medical professional: I’ve been seeing doctors and having tests done (I even switched my primary because he told me to just “wait it out”). So far, we haven’t been able to figure it out so I’d like to get some ideas.

It started right after July 4th: short spells of dizziness followed by extreme fatigue and complete loss of appetite. At first, I would have a day of symptoms, followed by 3–4 days of feeling relatively normal. As time passed, the time between symptomatic days shortened until the present condition, where I’m basically feeling tired and not very hungry most of the time, punctuated by the occasional feeling of dizziness (sitting, standing and lying down; it seems to happen randomly). I find that the computer screen sometimes makes the dizziness come, so I spend less time on the internet these days. I have days where I skip meals and I have days when I’m slightly hungrier, but I’m still eating far smaller portions than I used to. I have not been to the gym in 4 weeks (I used to go 3–4 times a week). I’ve steadily dropped 15–18 pounds since the symptoms first appeared.

So far, the blood tests have come up negative for diabetes and anemia. Liver and kidney functions appear normal. My primary says celiac and lyme disease are unlikely because I don’t have other symptoms. This past week, I’ve had a brain MRI, a cervical spine MRI, a chest x-ray and an abdominal sonogram. The results for those will be in at the end of next week.

I don’t know what those tests will show, but if they come up clear, what else could this possibly be? What’s the next set of tests to go for? My mother thinks I need to see an endocrinologist. My family thinks I might be depressed, and while I admit to having a history of depression, I’ve been through cognitive-behavioral therapy and have had good results. For some reason, this doesn’t feel psychological to me (and the symptoms are completely different from what I experienced when I was having panic attacks a few years ago).

It goes without saying, but this is messing up my life. I’ve been at a standstill for a number of projects because I can’t muster the energy to do them or because I feel lightheaded.

I’d like to know what questions to ask my primary, what specialists I should go see and what tests I should have done. And if my symptoms sound familiar to anyone. Basically, any insights or thoughts would be appreciated.