Diarrhea and then death.

My question is about the death of our Daisy Mae. We are staying in South Padre Island, TX for the winter. One morning I was awakened by Daisy Mae vomiting. She vomited several times that morning, probably 3-5 times. At about ten A.M. she asked to go out to go potty, and she had diarrhea that literally sprayed. It was dark brown, and there was some solid stool mixed in. She vomited once in the afternoon once after she got into the water. We had withheld both food and water. She had been sick once before in her 2 1/2 years, last spring when she ate some twigs from a birds nest. As the afternoon wore on she didn't seem to be getting any better. Usually a couple of hours after throwing up she would usually be showing signs of activity. So I took her to the vet at 4:00 because I didn't want her to go through the night without knowing what was going on. The vet checked her and gave her three shots: penicillin, Cerenia, DexaMeth, and Albon to start her on at 7 P.M. He said that if she had eaten some rotten hamburger, she would be O.K. But, he said, if she had eaten some rotten fish, or even anything out of the gulf, including sea-gull droppings sometime, it could be bad. When I was in the vets waiting area paying the bill Daisy threw up the most awful sulphurous smelling liquid I ever smelled. And I would guess there was at least a half pint to a pint. I took her home and we gave her the oral suspension of Albon at 7 o'clock along with a little water by syringe. I held her for a couple of hours and carried her out to go potty at 9:00. She just sat down and didn't do anything so I took her back in and laid her on the sofa. At about 10:30 I smelled that same smell as I had when she threw up in the vet's waiting room, so I went to check on her. She had died. When I was holding her my hand was over her temple and I could feel her heart racing, and her breathing seemed to be labored. When I picked her up to move her off the sofa, she had bled rectally and some from her mouth. We have another Shi-Tzu that is 2 1/2. We walk them twice every day, regardless of the weather, and they are always on retractable leashes. If they put their snoots down we are over them watching. We never take them down to walk on the beach. The only thing I can think of prior to her getting sick is that she got into a weed patch at the side of the street ditch and had something in her mouth. I yelled at her and she dropped it. The next afternoon she got something again and I stuck my finger in her mouth to get it out and could find nothing. I know for sure she wouldn't have had an opportunity to get into anti-freeze or any of our medications, aspirin, etc. An M.D. friend of mine said occasionally he had a patient whose bowel got twisted and that could have been what happened, but I don't see that causing the bleeding and the awful smell. I am not looking for an online autopsy here, but am still trying to understand what happened. We have the other Shi-Tzu and fortunately by the time we get home April 3rd the breeder where we got Daisy will have another litter, by Daisy's mother, which will be her final litter. We get first pick of a female. So, I am just trying to understand what it could have been that would have attacked her gastrointestinal tract enough to cause the vomiting, etc. and that could have killed her in about 18 hours. The vet told me if she had "paint the wall diarrhea, or was bleeding orally or rectally to get her back in there. Well, I think I should have done that right away in the morning. But, both dogs have been sick before, vomited, and in a couple of hours were running around chasing each other. So, after running to the vet with fecal samples and being told that is was nothing serious several times, I guess we got lax about the seriousness of the situation. Perhaps it would be worth a reminder to other owners of small dogs that their digestive systems and other organs are small and vulnerable. Throughout her whole ordeal the only time Daisy whimpered or acted uncomfortable was to go out when she knew she was going to have diarrhea, or when she begged for water, which we limited. Thanks for any insight you can give us.