It may not mean anything other than he played so hard he traumatized his bladder. When I was a vet tech we saw alot of dogs who had exercised heavily and had blood in their urine. If it only happened the first time after the exercise, the vet generally did nothing except told them to watch for a urine infection (increased frequency, more blood, increased thirst). I don't remember any of them having subsequent problems but most were hounds that had just gotten back from a hunt.
Thanks everyone. I called the vet and they were closed but gave an emergency number. I called the emergency number and the lady asked if there were any changes in his behavior. I told them no and said we were just playing fetch with Trooper. She said to monitor him and see if it keeps happening. Then she told me to call back if it gets worse or just call our vet on Monday to have him checked. So far since that one pee, he hasn't peed anymore blood. We'll keep watching and call Monday to see what they think.