Hopefully things have settled down and gone back to normal in your apartment.
It could be that your cat was trying to tell you something about that stray cat outside your apartment. Something that he could sense but that you could not. Perhaps the cat is FeLV or FIV positive, or even sick with FIP.

It is always a good idea to keep your cats indoors and not let them associate with unvaccinated, untested stray cats. Your cats will live longer, healthier lives and you'll save a ton on vet bills.

You'll also avoid issues like this one.