light brown smudge on my belly

How long have you had this light smudge on your abdomen?  Have you ever tanned in the past?  Have you had an injury to this area recently?  It could be an old bruise.  As long as it does not hurt, I would worry too much about it.  It sounds like a birthmark to me, but you would have noticed this a long time ago if it were.  Just keep an eye on it.  Hopefully, it will fade in time.  If it does change in color or gets any larger or changes in texture, have a doctor look at it.  Good luck!