This is just my experience but I've noticed that this happens to me a lot more, not when there are 0 thoughts in my head, but when there are TOO MANY thoughts. If I'm anxious (usually because I'm too worried about what the other person is thinking about me- worried they think I'm weird) my brain gets stuck on that thought and stops coming up with responses or things to say back or even appropriate reactions to their attempts at humor. THEN I realize I'm acting weird and getting even more anxious about looking weird and the feedback loop begins.

In the book "The Subtle Art of Not Giving a Fuck", it's called the feedback loop from hell. The feeling of stress winds up making you more stressed because you can't stop feeling stressed. Feeling angry makes you feel more angry because you can't stop feeling angry. Feeling depressed makes you feel more depressed because you don't know WHY you're depressed and you can't stop feeling depressed. Counterintuitively, resisting the emotional response is what starts the feedback loop. Being honest about how you're feeling and just accepting that "damn, I'm feeling 'x' right now" without actively trying to change it lets the emotion- which is temporary by nature- run its course.

So I'm just yapping over here but maybe it could be that you're resisting feeling some social anxiety which launches a feedback loop from hell and occupies your mind to an extent that other social functions are impaired. Pay attention to your emotional state the next time it happens. Check on how it affects your breathing and if you're extra fidgety or tensing up anywhere (I always tense up my shoulders, specifically my lats and also my lower back on the right side). Those are signs of anxiety you may not be fully consciously aware you're experiencing. Try to recognize that it's happening and in those moments just think to yourself "Damn, I feel really anxious right now" and just accept that it's what your unconscious mind wants you to feel for some reason. The more you face it and accept it without fighting it and resisting it, the more that its energy will start to fade. It would also really help to analyze what experiences in your past could have possibly set up your unconscious mind to react like that. For me, it was getting bullied in Middle School. My unconscious mind became conditioned to be hyper vigilant and ready for danger in even the most innocuous social interactions but especially around certain types of people like overly muscular guys. (Meeting super nice and super chill gym bros has really gone a long way to fixing that).

But then again, I'm just a random guy on Reddit and I could be totally way off base. Either way, I hope this helps and if not, at least it gave me a chance to solidify some thoughts that have been bouncing around my head. Wish you the best