Just so you know, this condition can go away. I had many similar experiences. I am male with a frame that is meant to support a lot of muscle ... none of which I developed. In high school, I wore a 28" waist, weighed in the mid 100s and should weigh closer to 200. I think for 6'4" I was supposed to weigh exactly 200. Now, I weigh about 210 or so with a approximately 35" waist. I never developed that muscle to fit my frame, so I still look skinny in some parts -- but lack of activity and a slowing metabolism of middle age has caused that extreme skinniness to be a goofy "skinny with pot-belly."

So, don't know if this is good news or bad. I've never changed my eating habits, though. I eat copious amounts. (And I don't really gain weight very fast... I've come to my current weight after 10 years... And maintained it for the last 5.)