I'm so sorry she passed. I can't tell you this is what happened to your pullet, but I had a few grow outs from a specific breeding pen I had bought and hatched eggs from and they did not do well in the heat. The one would get pale and pant and be so weak, I'd cool him down the best I could. On cool days he did better, so I kept him with cool foot baths, cold treats etc but I did lose him. This line had a genetic heart defect, they had very small hearts.
It is possibly your girl had something like this which caused her to be susceptible to the heat. As
@TwoCrows said, she knows you loved her and did all you could for her