Welcome to Dogsaholic! I love the pictures that you have attached to this comment. These are great dogs with an equally great pet parent. I know where your hunch is coming from regarding the dog food, and it is a highly likely suspect especially if there are no significant changes in your routine over the past couple of weeks prior to this incident. It might be a bad batch of bags. What you can do for the meantime is to continue observing, and follow the current course of management that was previously provided for your GSD. They are resilient dogs in general and I believe he will become better. Let me know how we can help you further.