I would be a bit concerned - and would suggest that letting a vet take a look can be a good idea.
Specifically, the following possibilities come to mind:
One is that possibly the constant pounding of leash walks might be causing her a little discomfort. At 8 months old, she is not yet finished growing skeletally, and still has soft areas at the ends of her long bones (the growth plates) which allow for growth to occur. Being still soft, however, they're still vulnerable - and constant pounding is one of the things that can put pressure on the growth plates. Running around like a lunatic off-leash won't - because the dog will take lots of short breaks and pauses to give herself rests when needed. Soo - depending on the length of your walks, that could be one possible reason (and you can fix it by either limiting the length of the walks, or yourself insisting on lots of short breaks).
Second possibility (and this is the one I'd consider most likely from what you describe) is that the problem is temperature and her ability to breathe. Brachycephalic (short nosed) breeds simply cannot regulate body temperature very well, and can easily run into difficulties in even slightly warm temperatures. My own dog does precisely what you've described yours doing - and in his case, it isn't just "brachycephalic syndrome" but a slightly elongated palate that restricts his ability to breathe well. It isn't quite bad enough to require surgery (it is a risky surgery, so only worth doing if absolutely necessary). Anyway - he is perfectly fine in cold weather, but when the temperatures start to rise, the natural enlargement of his tongue and other parts of the mouth (it is a normal cooling mechanism of dogs) tips the balance and the poor guy simply cannot take in enough air. And so - he has to sit down and rest. Your description makes me wonder if your girl doesn't have the same problem - and I'd suggest you let a vet take a look to find out.
Third possibility - a heart condition. That would likewise have more impact in the heat. You don't describe anything else that sounds heart-related. But I put this one in the possibles list anyway, since this is a breed prone to a couple of serious heart defects. It is likewise something a vet would need to be consulted on to determine if such a thing exists.
