Do you eat a lot of Sodium?
If you do eat more than 2000mg a day it might be that you are retaining fluids in your stomach area. Many people will hold it in their legs or all over the body but it te same with fat. Some people retain it on concentrated areas.
I'm not sure if that's the issue or not but you might want to try limiting sodium intake for a while and see if it helps.
My second guess is that you might have excess tissue there. Not just skin but connective tissue. You might want to ask your doc to test what it is.