why did my dog die?

Please can somebody help, my beloved dog died suddenly a couple of nights ago and I don't know why and feel terribly guilty for not realising how bad he was.
On tuesday we noticed he wasn't 100%, he turned down his food and didn't want to go walkies
On wednesday he was pretty much the same, walking ok around the house but off his food and decided not2 take him for walk in case he still wasn't feeling great
Thursday my partner thought he wld take him to vets after work if he was still the same but when he got home he ate some chicken and went for a walk ok, he was quite thirsty in the evening tho and just sat on the sofa. Thursday night he died. He had thrown up some brown fluid, not sure if blood, and was lying dead when we got up in the morning. It was such a shock and would really like to know what happened. I wish I could turn back time and have taken him to the vets sooner but really didn't realise how serious it was, he didn't seem2 be in pain or anything and thought maybe he just has a bad stomach.
He was a healthy dog however he did suffer from occasional fits for years that the vet said were possible due to epilepsy as he had no other symptoms.
Any help gratefully appreciated

[Aleda M Cheng, D.V.M., C.V.A]

I am sorry about the death of your dog!

It does not sound as if you did anything wrong.  His sporadic lack of appetite and mild lethargy could have been attributed to a mild stomach bug, or a cold.  He just did not have very dramatic symptoms.  The only serious symptom he had was when he vomited the brown liquid and by then it was too late.

The brown liquid could have been blood and could have come from his chest or abdomen and the causes could have been due to a ruptured tumor originating from his chest or abdomen, or from a fatal heart condition, or even from a blood thinner such as rat poison.  However, had it been a blood toxin, you would have seen symptoms prior to the brownish vomit, such as blood in his feces or in his urine or a bloody nose, mouth or bruising long before the vomit.

Unfortunately, we will never know definitely what was the cause of death without an post mortem exam.