I am so sorry for your loss. I know how painful it is to loose a beloved pet. I hope you know that there are plenty of us out here who understand what you are going through.
There is no way I can tell you for certain what the cause of death was, but the most likely cause of sudden death in cats is related to the heart and that would be consistent with what you described. They may develop and blood clot that then blocks circulation to a critical area, such as the brain or lungs, or they may develop an abnormal heart rhythm that prevents the blood from being circulated. They can also get heartworm disease that affects the heart and can cause sudden death.
I hope you have someone to talk to about this. There are several organizations that are set up to help people grieve the loss of a beloved pet. Let me know if you would be interested in more information about this.