Our puppy came home 2 1/2 weeks ago and ate nothing for the first several hours that she was in our house. She did manage to eat a full meal the same night though and has had no problem with food since. However, she did pee and poop very little for the first 2-3 days we had her... it was almost like she was holding it in! Now she pees SEVERAL times each day and poops 2-3x.
I think there is just an adjustment period and this is normal behavior as the dog is adjusting to it's new environment. I would only worry if it went on for more than a couple days.