I am having single nostril bleedings and can't tell why. Any idea why?

Lots of Causes. The top ten causes are on the ends of your hands. Nose picking is often the cause of single nostril bleeding. The trauma from nails can damage the soft lining of the nose, called the mucosa, and cause bleeding. Other things, like allergies and some medications, can also lead to nosebleeds. Your best bet is to avoid picking or hard blowing and perhaps use some nasal saline to keep the lining moist.