Dogs do not normally chew food any more than is needed to gulp it down. They rely on their stronger stomach acids to break down what is swallowed.
Pain after eating could be due to a pancreas issue even if you are not seeing vomiting or diarrhea.
You can read about this here
http://www.peteducation.com/article.cfm?c=2+1580&aid=335
This might also be due to an ulcer causing pain when stomach acids flow to break down food.
I'd plan a vet check up for him when your vet is available.
If your dog might have gotten into anything toxic (including toxic plants), 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 a rectal temp over 103.5F (normal is 100.5-102.5 Fahrenheit or 38 - 39.2 Celsius ), has gums or tongue that look 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, is having difficulty breathing, or is becoming more lethargic and non responsive to you as time goes on 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 an emergency vet if you need one
http://www.vetlocator.com/hotline.php
Hope this helps you!
Dog Expert:Rescue, Train,Breed,Care
30+ yrs dog home vet care & nursing, rescue, behavior&training, responsible show breeding, genetics