Welcome to Healthcare Magic,
I see your blood pressure is in normal range and if you had not any brain injuries in the past then most probably your neck muscles stiffness
and head heaviness is due to wrong position while you working on computer and vitamin D will not help. Try to maintain strait vertebral
column including neck and adjust computer screen accordingly , keep your legs on feet support with 90* between calf and thigh, shoulders relaxed. There can be also be involved your basilar artery
and cervical vertebras , in case of compression may develop mentioned by your symptoms. I would advice you cervical MRI
and Neurosurgeon consultation for definitely diagnosis.
I wish you good luck and speedy recovery,
Hope this helps and I have answered your query.