Why did my dog die after vomiting, diarrhea, and stomach issues?

Thank you for using Petco Pet Education Center, formerly Petcoach. I am sorry your dog passed away, it is always hard to go through that. From your description, it sounds as though your dog had a gastric dilitation volvulus (GDV) or 'bloat.' This is where the stomach flips upon itself. it is a life threatening condition that requires surgery in attempt to fix. Sometimes, the signs that occur when this first happens can be subtle and it goes unnoticed for some time before being treated. When the 'flip' happens, it causes a large amount of inflammation that the body must contend with and unfortunately, a good outcome doesn't always happen. It sounds like you did everything you could for your pet. Of course, it is hard to know tell what happened for sure based on your description. Since you are asking this question a year later, you may still be grieving. It may be of benefit to speak to the veterinarian who took care of Patch to discuss what happened. There are also counselors who work with grief over pet loss in your area that also may be of benefit to you.