Umm....underweight but you have fat....?
Hmmm.....
Thats odd....
Ok....this is what I think....because the lower abdomen is the
"core fat" or the "first ring fat" it means that you have just
started to accumulate fat deposit. (Lower abdomen is the first area
to store fat deposit before other areas). But since you are
underweight, it means maybe you have been rapidly eating high-fat
or high-sugar food, such that you are not gaining weight in a
steady and gradual manner but in sudden manner, and it results in
fat deposits in lower abdomen.
My suggestion is to lower your fat and sugar intake and space
your meal intake to 6 meals per day.
If possible, eat whole grain, unprocessed food, well balanced
and with variety.
Dont try to cut quantity because you are actually underweight
and you do need to add the weight.