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It is likely that the headache ( along with pain behind the eyes) could be due to migraine or chronic sinusitis. In the absence of symptoms of sinusitis such as headache along with cold/nasal discharge, the headache you are experiencing could be due to migraine. Migraine can have many variants and this could be one of them. The snapping sensation in the knees however is not related to migraine nor is the dental procedure causing the headache ( dental caries however is a known cause for referred headache).
Treatment of migraine is with analgesics ( acetaminophen) and prophylaxis is with various medicines such as Propranolol, Flunarizine. The dosage can be discussed with the physician.
I was wondering whether the Optician has measured the intraocular pressure of the eyes during the eye exam. It would help to know as we can then not consider narrow angle glaucoma as one of the causes ( though a more detailed eye exam by an ophthalmologist would be much better as more specific eye tests such as gonioscopy would be needed).
Though I cannot not be of more help without a proper examination , I could atleast give you a few threads which you can pursue such as a workup for migraine and to look out for narrow angle glaucoma as causes of headache with nausea.
If you have not had a scan done then a MRI scan of the brain would also help to know if there is any intracranial cause for the symptoms.
Please feel free to ask follow up questions if any.
Thank you and best wishes. It was a pleasure to have answered this question. A positive rating is what I aim for and we can continue to interact till you get a satisfactory response.A positive feedback and /or bonus are most welcome and appreciated. Views expressed are for information purpose only and cannot substitute a visit to an ophthalmologist