Hello,
I have gone through your query and understand your concern.
There are plenty of causes for lump in belly button also called
Umbilicus.
I suspect you are suffering from
Umbilical hernia.
It may be present there unknown and you were identified co-incidentally.
Many people have hernia and it goes unnoticed and very few people like you identify coincidentally or on symptoms.
You may have the hole or a weak area in the muscles of the abdominal wall near belly button.
If you are
Obese or having the family history of same problem, then it is mostly a umbilical hernia.
Please check whether it increase in size or pain while coughing or straining, If it so, then definitely it is a umbilical hernia.
This can be confirmed by simple examination and Abdominal
Ultrasound.
If it is a umbilical hernia, then better to do
Hernia repair.
Because this may cause a incarceration and Strangulation. Both are acute condition and severe painful.
Hope this helps.
Please write back for further queries.
Wishing you good health.