To check for channel imbalance if you're not sure, do the driver matching test here -
https://www.audiocheck.net/soundtests_headphones.php
Some songs may be mixed such that bass/vocals are not centered but there's no convention about putting vocals to the right side or anything like that. If reversing the headphones doesn't affect the direction of imbalance, and your pads, source and cable are fine, it's probably your hearing. As
@Lohb suggested, go to a doctor and check for wax blockage - it's quick to get your ears cleaned and the sooner you do it the better. Hope you find a solution soon