Mine do something like what you describe when it's hot even with fans running.
If it gets to 80degs in their area and it is climbing, I put in the frozen 2L bottles of water.
What do you do to cool them down?
Rapid breathing like they are panting, yes, that would make sense. I 've seen plenty of rabbits tremble when they were being held, too . Could either of those be what you are seeing?