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Spasm of muscle (say from tension) is most likely to be responsible.
However physical problems should be ruled out...
Though less likely, an ophthalmic (eye doctor) and Neurologic examination is a must to exclude any underlying disease. An MRI
of the brain is usually necessary.
If there are no clues, Psychological assessment and assistance will benefit...
Anti-depressants help by elevating the perception threshold / even if there is no depression
Wishing speedy recovery