Sitting here on the couch thinking back to this weekend, he was shaking his head a bit too. It was quite hot. Sweat in his ears??? We use have used a hack on him for 2 years, and his teeth were floated in March, so I rule that out too. Was at the farrier 4 days ago. Not limping or favoring a leg/foot.
The can chasers around here have dental and chiro checked if the horse goes from good to bad as fast as yours did.