Is it weird that I gain weight this way?

Im 18 & ive gained about 20lbs. I have always had a flat stomach. But even tho ive gained 20lbs my stomach is still extremley flat. I gain weight in my legs and buttock but im not really improportinate. My friend said that its something wrong because my upper body dosent gain weight and the weight is suppose to be evenly distributed. so my question is...is this normal? Should I c a doctor?