There are many possible causes of brain fog. When symptoms have been present for a short period of time, it is reasonable to consider various infections, and when the symptoms have persisted for 3 weeks, a specific possibility is that it could be related to mono. There can be certain findings on exam or a CBC to suggest mono, but no findings is reliable, other than specific tests for mono. It does not appear that a mono test was checked in the lab tests.
The next consideration would be whether there is a metabolic condition that could be causing the brain fog. You had most of the labs that would be appropriate for this, although there may be a couple others that would be appropriate to check. A comprehensive metabolic profile will include different components in different labs. It usually does not include a magnesium level, which usually needs to be ordered separately. It frequently also does not include thyroid function tests, although there are some labs that will bundle a TSH into the metabolic profile. If these were not included in the comprehensive metabolic profile, then it would be appropriate for these to be checked. It is better to check a magnesium level rather than simply taking a magnesium supplement, as excessive magnesium levels can also be dangerous. Another metabolic condition would be whether the blood sugar at times is too low, called hypoglycemia, causing the brain fog to come and go. A blood sugar would certainly have been included in the metabolic profile, but since the blood sugar typically is normal between episodes of hypoglycemia, a metabolic profile done at a doctor's office will frequently miss the problem. The most sensitive test for detection of hypoglycemia is to use a home glucose monitor and check the blood sugar when the symptoms are at the worst.
It is not clear that vitamin D deficiency will cause brain fog, but since it is the only abnormality on the lab work, it is reasonable to supplement vitamin D.
The other major type of medical condition that can cause brain fog are various neurologic conditions, but the usual first step in the evaluation for neurologic conditions is a neurologic examination, and this is apparently in the works.
A lack of sleep also can contribute to brain fog, and while most people know when they are not getting adequate sleep, there are some conditions in which the problem is not a sufficient quantity of sleep, but rather poor quality of sleep. In the latter case, the person perceives that they are getting a standard night of sleep, but if the sleep is analyzed, it is not getting into the deeper levels of sleep that is important for restorative sleep.
Certain mental health conditions also can cause brain fog, but there would usually be other symptoms of the mental health condition, such as depression.
Please let me know if any further information would be helpful.