Dried nasal discharge causes crusting inside the nose. When the crusts are accidentally removed or they come off on their own, it leaves an ulcer and bleeding occurs, it is common during common cold. Sometimes if there is acute infection of the nasal lining or the sinus, bleeding can occur. Nothing to worry. Keep your nose alkaline with daily douching or wash with a mixture of glass of warm water + pinch of salt + 1 teaspoon baking powder. Don't finger the nose or use tissues to clean nose. Don't blow the nose very hard. Take a simple anti-allergic tablet on advice of a doctor and relax.