... Where did this nosebleed come from?

The correct way to treat a nosebleed is to put your head forward, not back and pinch the bridge of your nose till it slows/stops. Do not blow your nose when you are trying to stop a nosebleed. It will blow out any clots that were starting to form and cause you to bleed more. A nosebleed can be caused by any number of things. If they continue to be a problem, see your doctor.