it is multifactorial
I have read your query. The reasons for fat getting deposited in the stomach or abdominal region are many. One important cause is heredity. It is the inherited pattern of fat accumulation from your parents that you also tend to put on fat on your tummy.
Other factors are the genetic make up in you and the fat cells are stored more in the tummy region.
Hormones play an important role as well. Increased stress hormone
levels causes one to put on weight more in the belly region.
Hence, the reason is multifactorial.
Your mom is right too, it is perhaps by birth or genetic reason in you.
There are chances for you to get a flat tummy. For that, you need to avoid refined carbohydrate
foods like white rice, white bread etc.
Exercise and staying away from stress helps too.
Wish you good health.