I can't say for certain, but this might
be part of it:
Narcs don't expose themselves. In a nutshell, that's what makes us narcs. Sex is the physical act of exposing oneself, so in part you've already done what you can't do, just physically instead of emotionally. Also, there are chemicals released in the body before, during and after sex. These chemicals are designed to do different things, but one thing they do is break down the walls we have. Studies have shown that in order for a person to reach orgasm, one must 'let go of all fear and anxiety'. This is a direct contrast to narcissism.
In essense, your body is fighting your brain. You've been given a cocktail designed to break down your emotional and psychological barriers in order to achieve orgasm. These don't leave the body the minute orgasm is achieved. Perhaps your 'recovery time' isn't as fast as you'd like.
So what to do? Try dropping hints maybe that after sex you just want to sleep? It's not as if you're looking to get into a relationship with the person you're with, right? So just make it clear you're not a big pillow talk person. Then, if they start talking, you can just say "I told you, I'm not a big talker after sex." and go to sleep, or home, or whatever.
As a side question, is it possible you're noticing it more because these chemicals are basically begging you to be open and your brain isn't equipped to do so? At these moments, you want to make some kind of connection but can't? Probably not, but I thought it an interesting possibility.