Hi glizj -
It sounds like there is a good chance that your dog has a condition called Exercise induced collapse in Labrador retrievers. Although we do not understand this condition completely, but we do know that it is hereditary. There is now a genetic test available through University of MN to diagnose this condition.
Here is a website from the University of Minnesota that does a really good job at discussing the condition
Here is another website that can also give more information about this
I hope this information is helpful. If so, please click Accept. If you have additional questions on this topic, please let me know.