capillaries in nose are very fragile and near surface, ie really easy to damage and really apt to bleed. You could have bumped your nose and forgotten about it, blowing too hard can break capillaries and cause bleeding later, picking can damage the capillaries and cause later bleeding, dry air can cause capillaries in nose to bleed, allergies can cause it...there are some more scary things that can cause it so you probably should have a doc look if it keeps happening or bleeds for a long time.