Definitely trust your instincts - if you sense something is wrong, something is wrong. I still haven't learned that lesson. More than one kid at the same time too. Diet is always the first place I look for ANYTHING. I'm no expert though!
My dog has completed his five days, appears to feel better, is eating more, and his breath is much better (it was so bad I could smell it as soon as I walked in the house). I'm not sure what to do now, if anything. He is getting old.