It is because of your constant focussing and may be increased screen time. i advise you to continue lubricating drops and also take breaks after every 20min for 2min
frequent small breaks and blinks will help you the most.
secondly it could be because of refractive error if you have any.
Next Steps
if it feels same even after precautions, i advise you to go for eye examination to rule out refractive error