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Do you use reading glasses now that you are over 40 years of age?
May be the prescription no. is different in both eyes. Since, it has been a year you should get your eyes checked again. You may be having presbyopia(happens around 40 years, may be delayed in myopics) by now and you could require reading glasses. That can explain strain. Another cause that could be causing problems is working on computer. This can acuse strain as well as dryness. For this you should use a lubricating eye drop like Refresh Tears 3-4 times a day. Thanks.
That could have been because of some allergy but since that is gone you need not worry about that. When you see an ophthalmologist/optometrist, rule out blepharitis as that may also be present. Thanks.
Follow-up questions, if any, are welcome.