Brief Answer:
several possibilities can be there..

Detailed Answer:
Hello XXXXXXX Welcome to HCM,
I've gone through your query and understand your concern,

There are several reasons behind this, you need to rule out stress at first..it can be in form of excessive work-load, lack of sleep, excessive consumption tea/coffee/alcohol (if you do), prolonged sitting in front of computer screen, recent history of dry eye/ any allergic manifestations..although some electrolyte deficiency (magnesium mainly) can have an impact, but not conclusive, so I'd not think about it for the moment..

If you can notice any of those above mentioned factors, just simply try to avoid it for the time being and observe..besides this, take good amount of rest, sleep properly (~8 hrs per day), avoid staying up too late and eat in time..(avoid junk foods and tea, coffee etc. for the time being)..

If you're a contact lens wearer, I'd suggest to use spectacles for the time-being, if not, then time for an Ophthalmological check-up to look in to it further..

Let me know if there's any further queries..

Take Care
Kind Regards

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