Like Red said, a slight nose bleed could be caused by a few things, some more deep-seated, and some no more than a localised problem. All the sneezing
could have ruptured a little blood vessel in
his nose, that's true. He also could have got some grass seed or some small irritation stuck up his nostril(?) That might have caused the sneezing fit? This is very common as dogs have that habit of sniffing their way through life! The vet can put a small scope up and take a look.