What may have caused my dog to pass away?

Could have been congestive heart failure, that causes bloating. So sorry about your dog.

The only way to know for sure on how your beloved dog passed away. Is to have a necropsy done by a vet. I am so sorry for what you are going through. Just remember and cherish the memories that you shared together. They will never ever fade and will always keep you strong. When my son Tobie passed away, I found a poem called "Rainbow Bridge" to help me get through it. And the dog you lost would want you to move on and love another dog. My prayers are with you always.

Rainbow Bridge

Just this side of heaven is a place called Rainbow Bridge.

When an animal dies that has been especially close to someone here, that pet goes to Rainbow Bridge. There are meadows and hills for all of our special friends so they can run and play together. There is plenty of food, water and sunshine, and our friends are warm and comfortable.

All the animals who had been ill and old are restored to health and vigor. Those who were hurt or maimed are made whole and strong again, just as we remember them in our dreams of days and times gone by. The animals are happy and content, except for one small thing; they each miss someone very special to them, who had to be left behind.

They all run and play together, but the day comes when one suddenly stops and looks into the distance. His bright eyes are intent. His eager body quivers. Suddenly he begins to run from the group, flying over the green grass, his legs carrying him faster and faster.

You have been spotted, and when you and your special friend finally meet, you cling together in joyous reunion, never to be parted again. The happy kisses rain upon your face; your hands again caress the beloved head, and you look once more into the trusting eyes of your pet, so long gone from your life but never absent from your heart.

Then you cross Rainbow Bridge together....

Author Unknown

I have a dog who has Pancreatitis. (She has a reeeeeeaaaaallllyyyy severe case of it) Usually if it starts acting up she will have to go to the dog hospital for 2 weeks. It looks like her stomach is bloated when it happens. If you dog is small then this might have been what he had.

He died from an absence of veterinary care. He's been sick for a month or more, as you have clearly stated. yet nobody has taken him to the vet. Therefore he is dead from something that may well have been treatable if caught earlier. When you grow up, never let your parents take care of your children as they are obviously subnormal members of the underclass.

it may have been a tumor, or the bloating could have been internal distress, or some sort of parasite.

The only way to know for sure is to have a necropsy performed.