What Causes Nose Bleeding After An Head Injury ?

My 5 year old fell about 2 feet and hit the back of his head yesterday. He has about a half inch gash and it bled for about 15 minutes. He did not lose consciousness or act any different. Today he got a really bad nose bleed out of nowhere when he was taking a nap. Wondering if I should be concerned or if he might have a concussion.