I appreciate your good intentions here, and I hope she did as well.
But you were making a lot of assumptions. For one thing, good follows tend to have very good fitness levels since one of the reason they are good is because...they dance a lot
Personally, I can dance nonstop -- and I do mean nonstop -- for 3+ hours. I do it all the time. I don't need a break at any time, the few seconds in-between dances are enough for me to recover. And if the music is really good, I get extra energy from it so I can keep going even when I'm exhausted
Also, I am talking about nonstop salsa dancing -- whereas at your party there was also bachata, which allows for extra recovery time (even if she was dancing all the bachatas, they are way less taxing than salsa.)
Next time, I would simply ask her when you have the chance and trust that if she is too tired, she will tell you so. (As a side note -- this is also why I am against people using the "I'm tired" excuse when they're not really tired and they just want to give an excuse, because it makes it less likely that the person being rejected will believe the "I'm tired" when someone really means it.)