Start with a visit to your primary care physician.  He/she can help sort out possible explanations.  Ask him about Chiari Malformation.

Damien Howell PT, DPT, OCS

It could be any number of things.  Seeing a good sports PT and/or chiropractor is one option because neck muscle strain could produce the symptoms you describe.  However, the mental fog is of greater concern.  Get a basline for your BP to rule that out and consider tests for CAD and a PFT if that is recommended.  Really, the list of possibilities are endless to include low sodium, sinus problems, inner-ear difficulties, blood vessels in the brain, current medications, locked joints, pinched nerves or compression in the neck, and even too much MSG in the diet. 

I would say follow-up for sure with trusted professionals in the best interests of your health. 

Wishing you all the best.

With apologies to the above posters, the influence of neck tension on the vagus and occipital nerves, and the associated feelings of pain or disorientation, are legendary among massage therapists. We see it every day. However, as you have found, addressing the symptom is not as productive as finding the cause, which is likey to be postural. Again with apologies, as Jasko recommends, I would visit a physical therapist for a postural analysis before handing this over to any symptom chaser, regardless of medical pedigree.

Of the differences between man and ape that are conducive to long distance running, one of the most striking is the ability of the head to move freely fore and aft in counterbalance to running motion, courtesy of the occipital portion of the skull and associated musculature. This back and forth movement has been credited by anthropologists with the great increase in mobility and migratory range of modern humans over previous hominids, by reducing the energy expenditure and muscle strain required to support the head during prolonged vigorous ambulation.

While this difference is mainly structural, people who spend a lot of time focusing on tasks in front of their field of vision, such as driving, watching TV, or working with almost anything (especially computers), tend to bring their heads and necks forward in an ape-like posture that is hard to shake off. Running, or doing any vigorous exercise, while supporting most of the roughly 8 lb.weight of the head in front of the center of gravity, is extremely taxing on the neck and shoulder muscles in the area you describe. Pain and tightness are the inevitable result, affecting the many nerves there, hence your symptoms.

If you would rather tackle the obvious problem yourself, there are exercises to help retrain neck posture. Good luck conquering this common and easily solved problem.