This is purely anecdotal so please don't panic...
My 10 yr old dog started showing very similar symptoms to yours - after running fast her back end would just give out and she would be incapacitated for a few mins and then right as rain again.
Like you, I thought it was due to the heat.
Then it happened just outside the vets surgery after we'd been in for her booster jabs and health check. I took her straight back in and the vet examined her while she was still having her episode. Apparently her heartbeat had slowed down to an abnormally slow rate.
(Bear in mind that we had 5 mins earlier had her health check, the vet had listened to her heart and it sounded fine.)
So we had lots of further investigations and it turned out she had a heart condition - the vet said she had probably had an underlying murmur her whole life but so low key as to not have been picked up or caused any difficulties before.
But by now she was getting worse through age and wear, and her heart was enlarged. Her condition was managed on medication for several months but unfortunately she went downhill quickly after that and she was put to sleep about 9 months after diagnosis.
She was almost 12 years old when she died - she was an old dog and far less capable of coping with her condition than a young dog.
I truly do not want to upset or panic you. It could well be something different in your case - also your dog is young and strong and if she has a heart condition she could live with it quite happily for a long time. But I do think you should push to get her heart properly examined, so that it can be either ruled out as the cause of her symptoms, or she can be started on the appropriate course of treatment.
Just one more point - my dog's treatment cost our insurers thousands over those few months - please get some pet insurance NOW, before you get a diagnosis (insurance will not cover pre existing diagnosed conditions). It is so worth the monthly premium.
Wishing you and your dog all the best xxx