Watson, golden lab almost two now, has been on Blue Seal Pork and Barley for over a year. He does very well on it, only poops twice a day, usually fairly firm. Almost no smell, and no gas.
I moved recently to the city and found Kirkland at Costco. The ingredients are very similar, i.e. no corn, wheat, gluten or soy. So I tried to make the switch since both the price and ingredients are very close.
We chose the adult Chicken and Rice. Started out by mixing half and half, and he really loves the new food. Eats it right up where as before he would eat a hour or so after we put his food down. Everything seemed fine for 3 or 4 days, but now he has the runny poops. Like pudding, and he has to go frequently. Last evening we were out for a few hours and when we got home he had had an 'accident' which he hasn't had since he was 6 months old. I felt terrible for him.
This morning he had to go out at 1:30 am. Diarrhea again. He swats and does the 'wobble' trying to go, but seems really uncomfortable. This morning he woke me up at 6:00 am. I leashed him and went for our walk up the street and he stopped and pooed right there on the side walk (which he never does). He went twice before we got home.
I took him out again around 9:00 am, and still the same. He had finished off his food from last night, but I didn't give him breakfast, just a piece of burnt toast.
Now he is laying around, totally lethargic. Oh he did get in our lap this morning for his belly rub and mutual wallowing (he does that when we kiss

) but now he's flat out on the floor and hasn't moved since.
Is this normal when changing foods? Did we do it too fast? How long before the change in diet affects the dog like this? Or could it be something else? He doesn't have a fever, but I did put Advantage on him two days ago.
Any advice would be appreciated. Thanks in advance.