Neighbour's cat died and my cat acting strange?

My neighbours cat died this morning and everyone reckons it was heart attack since there wasnt any marks or blood on him, he was fairly young too about 5 or 6, really sad but the question I wanted to ask was a day or 2 just before he died, my cat has been a bit scared to go up our garden and is spooked a bit by something . The neighbours cat's body was found at the top of their garden so is this just a coincidence or is there something that has killed their cat? I had a look myself but couldn't find anything. So worried about my cat, good thing is she doesn't go out too often whereas the neighbours cat was out 24/7 and loved outdoors, r.i.p little guy.