This has happened twice with my 2 year old female Brittany. After 2-3 hours in the field she just shuts down, quits hunting and wants to lay down. She is a big running, hard charging Brit. Only knows one speed in the field and that's "balls to the wall". But she is also in tremendous condition. 33 lbs. of muscle and bone. The food she is on is 31% protein and 18% fat and she gets more than the recommended amount for her weight. I also give her 1 tablespoon of Coconut Oil with her food. I haven't talked to my vet yet but probably will do so. I've had hunting dogs for 44 years and never seen this. Anyone out there experienced this and have any insight? Thanks.