Sudden Death in my 4 year old kitty

Tonight our almost 4 year old kitty Gibby died unexpectedly. He was fine all week and all day. Acting like his normal self. Eating, meowing, being a lover. When I was making dinner I heard loud thumps in my son's room I said what is that and my son said my laptop? He was in the living room. It sounded like his laptop being thrown on the floor I ran in and my kitty was having what looked like a seizure on my son's floor. He was on the top bunk sleeping a 1/2 hour ago. He must have fallen off the bed from seizing? I was so scared I thought he might be ok after he stopped convoluting. But then he just stopped and stopped breathing. His body looked like he was breathing which I think he was until one big deep breath and then nothing. I started blowing in his mouth but it was too late. I'm so sad even worse my son and daughter both seen this and seen his dead body. I held my son as he cried himself to sleep. I don't know what could have been wrong? I was in the process of getting him neutered I know at almost 4 that's late in life. Also I think he may have had worms in the litter box I noticed white specs I thought maybe worms. So he was going to get that taken care of too while being neutered. I don't think round worms can kill a cat can they? 
Sorry this is so long I'm in shock and so upset. Has anyone else had this happen? I read a couple articles on here about sudden death in a healthy cat. My cat never had seizures. I don't know what happened? Just very sad.

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