My 2 year old lab has been acting funny for the past couple of days. Typically hyper and always upbeat, she's now lethargic and withdrawn.

I changed her food from Iams to Kirkland "Super Premium" (which includes lamb) at about the same time she started acting differently, mixing it with half of her old food. Is this most likely the cause of her schizophrenia?
Upset stomach possibly?
If the change in her behavior is this striking there's something wrong. If you only did the half-and-half feeding for a day, it may have been too fast.. I usually start with 1/8 to 7/8, then 1/4 to 3/4 the next day, etc..
A really fast switch can lead to pancreatitis in some individual dogs, which is pretty serious...
Time to get her to the vet.
Jo Wolf
Martinez, Georgia