The PD of a person can be changed when he/she is looking at different viewing distances. Normally the more close-up you are looking at, the more your eyes will converge . So the PD may not be fixed.
However , if you feel eye strain with particular glasses, it could be a number of factors leading to this. PD of the lenses not matching your eyes at that viewing distance, the wrap-angle &/or tilting angle of the frame, power of the lenses ( including the astigmatism and presbyopia power ) etc.
You are recommended to have a comprehensive eye examination together with both glasses and let our optometrist to fix the problem for you.