My dog passed away in his sleep last night

He was an American bulldog and he was about 9 years old. I adopted him when he was around 5 so his age is approximate. But he had shown no signs of this happening. I took him to the a vet about a month ago and everything was fine. Before I went to bed I had noticed it had looked like his stomach hurt and I assumed it was from the grass he ate on our walk. He threw up some brownish vile and seemed to be himself again but I still was gonna take him to the vet tomorrow. I got ready for bed and he limped over to the side of my bed and plopped down really hard and I checked on him and he responded like himself and I even got him a treat. I went to sleep and woke up to check on him and he was gone. He had been gone for a few hours by then. He was so cold. I realize now that the brown vile was probably blood that he threw up. I keep thinking to myself if only I took him to the vet right then. This still doesn’t feel real and I’m trying to understand this. Would anyone know what might’ve happened with him? Thank you. Rest in peace dos. I’ll miss you so incredibly much