"Head injury" doesn't mean the same thing to doctors as it means to laypeople. But practically speaking, whenever you have an unexplained nosebleed you should have it checked out. The blood may be coming from behind your nose. And even if it's coming FROM your nose, the nasal septum is supplied by 3 arteries, so whatever the cause, you can lose a lot of blood if it's not controlled.
See 1 more doctor answer