It could still be the change in diet -- once diarrhea starts from a diet change, in can take more than reverting to the original diet to get it back under control. This is where a bland diet (boiled chicken and rice, or low fat cottage cheese and rice, or boiled hamburger and rice, etc.) can help. Get the poop normal on the bland diet, then reintroduce the original kibble. However, unless it was literally that you added the canned food and the diarrhea was immediately the next poop or 2, I would take in a stool sample to check for parasites, including giardia, just to be sure. Also, if the bland diet doesn't help or the diarrhea returns when you add the regular kibble. And obviously if the pup becomes not happy and bouncy and playful and drinking normally.