Overload. Perfectly normal. For me, it's not so much that things sound the same but I get mentally tired of listening so intensely. Since I'm tired, I don't want to distinguish minute differenece in sound, and trying further just irritates me. Usually, I can just ignore the music, but sometimes I need to have silence. It usually lasts a few days to as much a week, and sometimes--rarely--more.
There's different angles to this. One is just getting tired of the stimulation. This might happen after a long concert or hours of being in a club, and I just want silence. Here, I'm usually listening, but not critically. A second is more mental, where I get tired of the mental exertion of thinking about the music or sound. This happens when playing music, listening intently to music, or at a meet, trying to hear slight differences in sound. Sometimes, I even get tired after listening to just one song--it happened to me recently on the subway after listening to Jessica by the Alman Brothers. Third, my ears start to hurt after too much loud music, which may be the only one of these where my ears are tired, rather than my brain.
It's probably worst at a meet listening to so many different things--short music clips and different equipment--and listening intently, concentrating, trying to discern minor differences. Something similar happens when club hopping and hearing all kinds of ugly music through horrible sound systems and yelling to hear each other. After a while, it all becomes just too much noise, and I want a nice quiet piano bar, or a quiet coffee bar (Irish coffee, usually). I try not to listen to too many things at a meet. I might have one or two things to do. At one meet, I had one goal, and compared well over a dozen amps to go with my current equipment. Only the amp was changed, not the source or headphones or cables or anything else. That's all I wanted to do because I know there's a limit. At some point, I stopped. You also need to save some brain bandwidth for interesting things that come up. So, I usually pick one unambitious goal for a meet, and maybe a second very small one.
I don't think our brain/ears are designed for this kind prolonged and intense listening. Imagine an animal hearing some noise it can't make out, staying up all night, listening intently, trying to figure out what it is, whether it can eat it or be eaten by it. The poor beastie would be exhausted by the mental exertion of all that intense listening after a few hours. Listening at a meet is kind of similar. If I buy that amp, will it eat me and my money or...