I am so sorry to hear about your loss. My heart sank when i read your story.
I am engaged to a veterinary tech., according to him the only way to be sure what the problem was is to do an autopsy.
What you are describing could have been caused by various ailments or a combination of problems. Only an autopsy will show what really happened....speculation will only cause more stress.
In my opinion however i suggest you remember the good times and cherish the memories you have. You cannot bring your pet back...but you can always remember the good times.
check the cemetery page on this site. it is very tasteful and it may be useful for you to place a tribute.
again I'm deeply sorry for your loss
best of luck