Sudden Death in our Yellow Labrador, 2.5 years old?

Hello, last Thursday our beautiful male 2.5 year old yellow lab died with no warning. He was fit and full of life. He excercised playing fetch at the park nearly everyday and would run and run. We were scheuduled to go to Florida on a 6AM flight last Friday, and dropped him off at the kennel where he was born, to stay the weekend. He LOVED the farm/kennel where he was born and we had taken him there several times when we went on vacation. Two hours after we dropped him off the breeder called to say that he was playing in the field and "dropped dead". We are just devastated. The breeder took him to our vet and there was no sign of trauma and his stomach contents were fine so he did not ingest anything. There were no signs of snake bites. His gums were pink and his eyes were clear. Our vet seems to think that he had a blood clot that dislodged to his brain. We did not want to have his skull cracked to find this out. We cancelled our trip and have been researching this all weekend. We were just wondering if you had any insight on this and if it's possible he was born with this defect. He was the picture of perfect health and had his heartguard and frontline faithfully. He was just tested for heartworms 3 weeks ago and had his heart checked as well. He was 64 pounds and VERY fit. Has anyone heard of this before?