Have you had it pierced? If so, and you are allergic to nickel (a very common allergy) and have put metal adornments on or over it, that could cause what you have described. If you have a metal button on jeans like a stud set button, the portion of the button that touches your skin could cause a reaction like you have described. If it has been pierced, and exposed to bacteria, that could cause such a reaction. If none of the above mentioned possibilities apply, then I suggest that you go to a Dr. To get an answer, and/or if you have an infected bellybutton, you probably should see a Dr. To have it treated.
If it is pierced you should know never to pick off crusties or to pull the dried crusties and the ring away from the actual hole in your navel. The proper way to remove crusties is by soaking the piercing in warm salt water, or plain warm water and wiping them away with a steril q-tip. When you pull crusties off it causes rips and tears thus the bleeding. Here is a proper care sheet.... www.bmezine.com I would also go back to the place you got it pierced and have the piercer take a look at it to make sure you don't have an infection going. If it is not from a piercing go to the doctor.
Your navel is one of the most sensitive areas found on your body. It should be cleaned daily when you take a bath. If it is not cleaned regularly then there will be a built up of dirt with causes your navel to itch. When ot itches you will rub it and this will cause constant irration, so it will smell and have a crustation.