Mostly due to Hygiene problem. Can be a infection
Hi , I am Dr Anshul Varshney , I have read your query in detail , I understand your concern.
Belly button is a closed area where it's moist in there and bacteria is trapped in there along with sweat, that don't ever get fresh air. Also that area is deeper then the chest and abdominal wall , so free debris and all impurities like sweat goes inside it.
Any smell or discharge coming from the belly button can be due to an infection. The most common causes of infection in the belly button are due to piercings, fungal infections, yeast infections
If you have redness
in the affected area, painful swelling, and curd-like discharge resulting in a smelly belly button, it is sign of infection.
In case you just have smell , and no other signs of infection , just clean it regularly with a cotton bud during bathing.
This is my personal opinion based on details available here. If you still have any query you may please ask me.
Dr Anshul Varshney , MD