Why did my dog die?

I have a query re. cause of death in older dog. She was a Yorkshire Terrier, approx. 15-16 years old, and had been on diet of Hills ZD food since summer of 2010, to deal with gastrointestinal problems (causing chronic diarrhoea and weight loss). This diet change had worked, although she never really regained all the weight and muscle mass she had lost, and still had problems with loose/runny stools, although nothing as dramatic as the episodes of diarrhoea she had previously.
She has been very gradually slowing up over the last 18 months. Then appox. 2 weeks ago her breathing pattern changed for short periods of time when it became quicker and slightly laboured, before becoming more restful again – she never appeared to be in any pain or distress when this happened. She also began to become disoriented – going to the wrong door to be let out, standing staring into space until her head drooped and she began to fall down. Again, these episodes didn’t last, and she usually became alert when her name was called. Otherwise she seemed comfortable – still eating (although appetite had slightly decreased and she was beginning to lose weight again), still barking, still interacting with family. In short some days were better than others. She had also developed an odour that I would associate with renal failure based on past experience, and had begun to sneeze regularly.
7 days ago the rapid breathing pattern stayed with her permanently, even when she slept, and periods of disorientation also increased. We realised she was winding down and that we might have to consider euthanasia at some point in the near future. Two days ago she died naturally and while we were with her. It was very quick and peaceful. I don’t know why this is important to me, but I feel the need to know what the final cause of death was, and if there was anything we overlooked or could have done. On the night she died, she was resting in her bed when she lifted her head and a few tremors or shudders (seizures?) passed through her body, before she settled back down again. A couple of minutes later, her breathing became loud and ragged and when I looked over, her mouth was open, tongue hanging out. She seemed to be gasping in breath. I lifted her up (her body was limp) and this rasping breath continued for a few seconds more before it stopped, she seemed to become alert again, stretched out her body and looked around at us. I thought she was ok and put her back in her bed, but then she took 4 or 5 gasps of breath with about 10 seconds between each one (so long that I thought each one was her last) before she stopped breathing altogether.
I realise that she was at the end of her natural lifespan and that you have to make a best guess at what happened, but I would very much appreciate your opinion on whether this was heart failure, renal failure, stroke, or if something else was overlooked by us such as Cushing’s disease. She did have swollen abdomen and some slight hair loss which we put down to ageing process and her dietary restrictions.
The last time she had bloods done was in summer 2010 when digestive disorder was diagnosed – at the time all were clear, no heart or kidney problems, or diabetes etc.