Sorry to hear of your sad loss.

I lost my 17 year old cat in the same way and i know how shocking it is. Cats with renal failure go downhill so quickly. My vet diagnosed renal failure in the same way as yours, ie by feeling abdomin and by how she smelt. She was also totally dehydrated. I was so unprepared for the diagnosis and asked if she could be put on a drip to hydrate her. The vet did this really for my benefit and took some blood tests to prove her kidneys were not working. The vet was right all along and 2 hours later I found myself making that horrible decision. I think this is such a common way for cats to go that vets know through experience what the results will be and try to save you the expense.

Dont feel guilty about anything, your decision was made through love x