Sounds like it's the food! Make sure you do a complete google search of your particular brand!!! If your feeding treats, do them , too!!
Check the expiration date on the bag. The other possibility might be mold.
I would return it and ask for a new bag.
Anything they could be getting into? Old food, garbage, plants? Have you used any sprays in the yard? Any new flea or Heartworm meds?
It was just a new bag of the same food.
Was it just a new bag of food or a completely different kind? Switching over to a new food has to be done slowly, by mixing the old & new kind together for approx. 7 days.
What kind of food are you feeding them now? It's possible that they're all sensitive to it.