Anecdotally, I can say your issues are consistent with Celiac disease. Headaches/migraines, depression and "brain fog" are some of the most common symptoms. Your B12 and D deficiencies are also suggestive of malabsorption of nutrients in your digestive tract.
It isn't necessarily Celiac (I have a bias here since I was diagnosed with Celiac/gluten intolerance in 2009 with symptoms quite similar to yours), but I would get some tests from a doctor who can look at and rule out GI parasites, thyroid issues, Celiac, Crohn's/colitis (especially if you have GI issues like gas/bloating as well.)
Also, if you have been in an area with ticks, get checked for Lyme disease--the symptoms you describe are also indicative of Lyme, and the "bullseye" spot on your skin from the tick bite doesn't happen to all Lyme patients.
Good luck!
Source: Not a doctor. Just scared of carbon monoxide poisoning. You know, going to sleep and never waking up.
My problem started when switched my job in April 2014. Before I was perfectly fine. The office space was not properly ventilated. Initially (from the very first day) I used to feel very drowsy in the office and fine outside. After few days the problem became permanent even I am not in office. I was feeling like mentally blocked. Also started feeling disinterest in everything. After a month in that office, I had to travel to USA for a month in the same company and there I got severe itching and doctors there gave me some antibiotic. After taking the antibiotic, my problem even got worse. Then I had severe headache and severe metal fatigue. After returning from the US, I had to go on unpaid leave. It was feeling heavy brain fog, feeling extreme stress and metal fatigue all the time. I had visited three neurologist. All of them prescribed anti depressant. I even took an antidepressant (setraline) and it made me even worse after taking two and half months. I had got few check up done in August including MRI. The doctor told me that the MRI is normal. I have also visited few ayurvedic doctors and the ayurvedic medicine worked to some extent. In fact I am continuing the ayurvedic medicine till now. The ayurvedic medicine at least not letting the problem to get more worse. I had difficines of vitamin B12 and D and taking injection for B12 and supplements for vitamin D. Later I consulted a psychiatrist but again no help. The psychiatrist got me the EEG test which was normal. On the net I found a guy ( who is not doctor by education) describing about anti oxidants and its role in brain fog. I consulted him and he prescribed different kind of stuff (mostly for antioxidant functions) that seemed to help a bit. At present I feel little better than but the problem still very significant. Neurologists have nothing other than antidepressant to prescribe and antidepressant has worked negatively. Right now taking Ayurvedic medicines, vitamin supplements and antioxidant supplements.
I have not got the sleep test done yet. None of the doctors I visited asked to me to do that. Right now I am planning to get that done too. Is there any test that I can get done?