MOre than likely, you don't eat enough calories (hence the skinny, lanky, ectomorphic body and very low bodyweight for height) yet the QUALITY of your diet is poor thus causing insulin resistance and abdominal fat storage. I see it ALL THE TIME.
If the quantity (calories) is poor (low), you're gonna weigh less/be skinny. This is simple physics and pretty straightforward.
But if the quality is poor (type of food/calories), you're going to store a lot of those calories - particularly in the gut area.
Eat more, but drastically improve the QUALITY of your diet. Eat VERY clean like you were on a strict diet, just eat a large quantity of clean, high quality foods in numerous, frequent feedings.