How old are your babies? Are they normally pretty active? Do they spend a lot of time out in the heat? How conditioned are they to the experience they had on the 40 min walk/dog park?
My Lexi can go forever in any temp and she's 6. Miles on the other hand pants really badly and gets really slow on the walk. (He's like an old man… we don't know his age, he was a rescue). If it is hot out, Miles will pant just going down the stairs. Lexi hardly ever pants. And then of course, if it's been a particularly long day for them, they both crash on the couch until they get hungry.
But I know how my girl is, if there's food- she's there. One time, she ate something (vet said it was a pill; I still don't believe her- we don't just have pills lying around all over the floor!) and she threw up and then laid on the bathroom floor. I got a treat out and told them to both to get in their crate- when Lexi didn't even lift her head-- I knew something was REALLY wrong and drove straight to the vet. (Her normal reaction is to FLY into the crate for the treat.)
It's all about knowing your dog. How long have you had them?
I hope she's feeling better soon!!