You didn't lose that rally. Your lungs did.
Long point, tied score, the ball keeps coming back, and somewhere around shot six, you realize you've been holding your breath.
Your legs are fine. Your hands are fine. It's your breathing that broke down first, and it cost you the point, right in front of your partner, your opponents, everyone on the next court over.
It's not you being out of shape. Pickleball's short bursts and short rest windows catch fit players off guard too, it's a different demand than steady-state cardio, and it shows up at the exact moment everyone's watching.