Dog fainted while playing fetch?

I was throwing the ball for my bullmastiff and all of a sudden is back legs locked up and he fell over. His eyes never closed, and it did not convulse but he had a vacant look in his eyes and seemed to be dazed...anyone had this happen? He just had a bunch of blood work done about a month ago and they said his organs were in tip top shape. Seizure, heart attack, overheated???