Older Jack Russell mix suddenly acting tired, maybe because of diet change?

My dog, Bub, is a 10-yr Jack Russell mix. Been on Nutro Natural for a while (3yrs?) but bc he's had some gas problems and his abdomen felt hard at his otherwise normal check-up, the vet told me I should try switching his food. Picked out Orijen, highly rated, high in fiber, and we've been slowly switching for 10 days now. Bub has all the sudden slowed to a near stop. His gums are pink, no vomiting, no diarrhea, eating and drinking just fine, stool looks normal, no coughing or anything else, no change in habits, but he's just not happy. Won't wag his tail, even for a treat, and walks really slow and sits down all the time. Today we went outside to do the business, and he had to sit down 3 times on his walk back up to the house, had to coax him to move him. I've poked and prodded, get no negative reaction. Does have some arthritis in his back knees, but usually he holds those legs up if they hurt. This is different. Have a call into our vet, but I'm really concerned about him. Any ideas?

Asked by Member 1048374 on Aug 24th 2011 Tagged depressed, dietchange, lethargic in Other Health & Wellness