My 3 year old son just had his first nose bleed what could of caused this?

Normal / common. Kids commonly get nose bleeds when they are exposed to airborne irritants (smoke, fragrance, fumes, dust, allergens) or dry heat. They often awake with a bloody pillow after rubbing their nose into it during sleep and having a bleed. Nasal saline wash or jells and humidified air help reduce the frequency.Avoidance of irritants is the primary method of prevention.