This is a well-known phenomenon among people with autism and ADHD, a population that significantly overlaps with the Ehlers-Danlos and fibromyalgia crowd mentioned in the post by u/Onikenbai, but it affects all humans to varying degrees of severity.

The best way I've heard it explained, that jives with my own lived experience as well, is that our brains are highly sophisticated filters that tune out the majority of our sensory input.

Think about focusing on a single sound in a noisy room, or identifying one flavor in a complex dish. Now imagine the opposite where you're simultaneously focusing on EVERY sound in a noisy room, AND the food you're tasting, while feeling every square inch of cloth that's touching your skin, the weight of your body on your joints and muscles, every scent in the room, the air currents pushing on every exposed hair....

You would be in a full-bore sensory meltdown and someone would probably find you rocking back and forth in the fetal position on the floor.

That's what our brains try to prevent every second of our lives, and it is a very taxing, resource intensive process. When we're tired, our brains are trying to do the same job with fewer resources, so the filter isn't as effective. Sleep deprivation, low blood sugar, dehydration, and simple end-of-the-day fatigue all stress your brain's ability to filter out unnecessary information. The result is that everything is a little louder, a little brighter, a little more intrusive than it would have been earlier in the day.

A fun example of this that could happen to anyone are the shingles scars on my left shoulder and chest. They itch constantly due to damage in the spinal nerve branch that feeds that part of my body. There's nothing wrong with the skin, but it always itches because of the damage to the nerves.

I can't feel it right now because my brain has gotten used to it and filters out the sensation, like how you can't smell your own house unless you leave it for a while. If I miss a night of sleep though, those scars itch incessantly and there's nothing I can do about it except getting some sleep to recharge my brain.

The only other thing I can think of is if there's more background noise from outside the theatre earlier in the day that raises the noise floor and makes the sound system sound quieter. I imagine you probably have pretty effective sound insulation in a professional theatre though.