Possible Cause of My Dog's Death?

On Sunday I had to make the hard decision of getting my 15yr old Jack Russell Terrier put down. He seemed to be fine that day, he came up to me looking for his usual cuddles and then headed upstairs to lay down.

He then came downstairs and my dad had noticed that he had peed on the living room floor. The door was closed so he could not get out; he would usually only pee in doors if he couldn't get outside. My dad let him out into the back yard and about 10 minutes later rushed in to tell me that something was wrong with him.

I went outside to find him laying on the wet concrete looking like he was finding it difficult to breathe, it also looked like he was straining, and trying to push out a bowel movement with no joy. He then vomited a very tiny amount of chicken that he had earlier that day. I tried to get him to move, but he would not and when I did eventually get him to stand he would stagger and then fall back down as if he had no power at all, especially in his back end. I tried getting him to eat or drink something, but he was not interested and continued having these symptoms for a while. I rubbed his tummy and he continued to look like he was contracting his tummy and eventually a tiny bit of diarrhoea came out, but he did not even have the energy/power to stand up.

We had to call the vet out of hours and met her at the practice, when we were in the car he was looking around him and eventually buried his head into my chest. When we got to the vet she examined him and said he was really unwell, his temperature was low, he was in pain when she examined his abdomen, his blood flow was bad as his mouth and eyes and colour was very pale and it took her forever to get a heartbeat and when she did she said it was very faint. By this stage he had worsened, he tried to stand a few times, but couldn't and eventually he was just laying there. When my dad asked her about tests she said that he is an older dog (15years) and the outcome would more than likely be very poor.

For weeks now he had been sleeping most of the day, laying in places he wouldn't usually lay. He slept in my bed with me and every night/early AM he would look to go downstairs for a drink and he would spend ages getting a drink then go outside for a pee. He seemed to walk about the house and not know what to do with himself, go up and down the stairs and at times kept out of sight, but other days he seemed to have more energy. We thought this behaviour was just because he was an old dog. I just don't know what happened and I'm finding it very hard to get passed this, I feel that I have failed him in some way. If anyone has been through a similar situation and has some insight into what may have happened it would be much appreciated and might help to put my mind at ease. The vet said we were very lucky to have had 15yrs with him, which I know is true, but he was my best friend.