A nose ring can develop an odor for a couple reasons. During the healing phase, there's a mix of wound fluid, blood, and dead skin cells that can create a bit of a smell. This is completely normal and should go away once the piercing is healed. Even on a healed piercing, sweat, soap, or shampoo can build up around the jewelry, causing an odor.
http://flashugnews.com/why-does-my-nose-piercing-smell-know-about-the-causes/
http://flashugnews.com/why-does-my-nose-piercing-smell-know-about-the-causes/