Dehydration, emotional or physical stress, and poor sleep hygiene can all result in reduced blood flow to the broca region of the brain, which is crucial for speech and audio processing. This is most frequently identified due to speech issues like gibberish or slurring (i.e., some of the more extreme/exotic symptoms of ptsd), but it also can manifest as hearing issues. Google up Broca's Aphasia for more information on that. Due to the fleeting and temporary nature in which most experiences of altered hearing occur in otherwise healthy people, this connection usually only registers with people who have another persistent issue which is the underlying cause. You, a little bit outside of the norm with a trained ear and all, seem to have cued in on this. Right idea, right observation.

Try to relax, drink water, maybe try resting the day before a show. Avoid the salty foods, maybe do some pushups to keep your heart-rate going between shows. The best solution is all going to depend on how you're built.