They say dry gets stale and icky after 3 weeks of it being open. I don't know if that's accurate because it's certainly in a lot of people's best interests to get you to buy bags before the old ones are finished...
Please don't let this episode deter you from Wellness. It's really a great food.
All I can say is that the gastro can be caused by a number of things. Anything that might disrupt your cat's immune system (like stress, cold, etc) could cause your kitty to be more vulnerable to bacteria...