It is perfectly understandable that you are having a hard time processing what happened so suddenly to Wally. When a beloved pet is so dependent on the owner, a loving and caring one like you, will question yourself and want to answer the ‘what if’ question. You know that there is not always an answer, but such a common sense answer cannot always deal with emotions.
You know that many of us here have been down this particular road ahead of you. My own little Jeeves, at 4 months old, had a vet appointment after I noticed his apparently swollen tummy. It was for the next morning but he did not make it through the night. So whatever was wrong, took him in very swift time. As Maz said, they are so small that their little bodies can be quickly overwhelmed.
Your husband talked to CC about hamster heaven. A lovely idea. There is also the rainbow bridge that many think of as the crossing point for a departed pet. On the other side, they are restored to full health and play happily there with those that have gone before. It is a comforting thought and strangely, many of us, including myself, have seen a rainbow in the sky soon after, as if a message of comfort is being sent.