If you don't get headaches when the weather is cool, then I would agree that it is heat related. I have a hard time running in the heat too, it saps my energy. I try to run early before it gets too warm, and to run in shaded areas.
Niko: American Showline GSD 5 years old
Rosa: American Muppet Dog (GSD/Border Collie mix) 5 years old