"Around Annapurna" with Mountain Travel in the early 1980s. I was in terrific shape (at sea level) and thus very surprised at how hard I was breathing after running 100 meters on the Thorong La Pass at 18,000 feet.
That's the kind of experience that makes one appreciate what Tim Macartney-Snape did, practically speed walking from sea level to summit Mt Everest w/out suppliementary oxygen.
Just another 80s trekker passing along a hello. I had the same experience as you describe nearing the top of Kala Patthar. Trekked outside of Pokhara too but did not do the circuit. Maybe we passed along the trail though ;)
Altitude sickness is also very unpredictable and sometimes hits the fittest people the worst.