They do love hunting and can run themselves into being overheated even if it seems relatively cool to you because they will push on even if very tired.
If your dog might have gotten into anything toxic, appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has gums or tongue that look bright red (overheating), white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, has a rectal temp of over 103.5F (normal is 100.5-102.5F), or is becoming lethargic and non responsive to you then you may want to get the dog in to see an emergency vet if one is available in your area.
This site may help you locate one if you need one
http://www.vetlocator.com/hotline.php
If you suspect it might be overheating from working too hard you can read here about some home treatment
http://www.parkwayanimal.com/information/general_information/preventing_heat_exhaustion.htm
before heading to a vet for more help.
As the dog has been working hard you might try a little bit of sugar with the dog such as pancake syrup or karo right on the tongue but more important is checking to see if the dog needs cooling down.
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics