Hi, popo. Welcome to the board.
Was your dog bleeding from both nostrils or just one? Did the bleeding stop on its own? Has he been having any kind of discharge from his nose before this?
The dogs in this thread had recurring nose bleeds. If this is your dog's first, it's hard to say what might be going on. He might have bumped it on something or perhaps his nasal membranes are dry. (Sometimes running a vaporizor or bringing your dog into the bathroom when you shower can be helpful.)
Certainly if he starts to have more frequent nose bleeds, you'll want to get a little more aggressive in getting a diagnosis. If you have access to a veterinary teaching hospital, they would have specialists available, which might help in getting a quicker diagnosis. So if the bleeding begins again, you might want to ask your regular vet for a referral.
I know first-hand what you're going through
Scary stuff, but try not to jump the gun. Maybe it will turn out to be a one-time thing