Welcome to the sauna of SE Texas! Shorter marks and shorter sessions are the rule. Shade and air flow are your allies. Learn to recognize overheating symptoms and be prepared to throw the dog into the car with the AC if needed. My first thought on your example was EIC. I've run my dogs to the point of being pretty hot sometimes and I've never seen that reaction. It does smack of typical EIC episode though. Cheap and easy test to gain some peace of mind.
Good luck and be safe!