My dog went to the vet today (she had a vaccine due), and the vet felt her abdomen, didn't feel anything unusual, and my dog didn't seem to feel any pain.
The vet said that since she's such a young dog (mine is 11 months), it's probably just her food. She said to call her if it gets chronic, and switch her to her old food in the meantime.
Also, the vet said it can take a week or a little longer for the new food to "sink in" to their system, so that means they could suddenly start vomiting even after they've been on their food for a week with no apparent problems.