How do I determine if I have a faulty brake master cylinder or if I just got my brakes too hot and had brake fade?

Did an autocross event today and it was hot as balls. I had a dual driver, so my car was running twice as often as other cars, with almost no breaks between runs. And the final section of the course was very high speed and required a very abrupt stop after the finish. On my last run, while I stopped at the starting line, my brake pedal sunk to the floor, so I did not do that run and let my car cool off. When it was time to head home, my brakes felt fine.

Only reason I'm worried it's a master cylinder issue because this is the second time I've experienced a spongy pedal in the last couple weeks, but last time it was during normal driving. I bled the brake fluid and that seemed the fix it. Fluid is very new, too.

How would you anons further diagnose? Am I just worrying over nothing and brake fade doing some autocross is normal?