extremely skinny and healthy but big stomach

Ok, so I have this problem. I am extremely skinny everywhere. too skinny almost. Except my stomach. It has always been my problem. Even since I was little. I looked at young pictures of me and I still had this problem. I am 17 years old. pretty active, and I consume only around 1500-2000 calories daily. I learned that the average teenager is supposed to consume about 3000 calories per day. I eat healthy food and I am always being health conscious. I use to run 4 miles each day and never saw any results. i do eat a lot of pizza (around 4 pieces every 2 days) but i dont understand why it wouldnt show anywhere else. I feel like its not the problem. I always assumed I had a fast metabolism because of being so skinny. The only reason I am consuming so few calories daily is beceause I dont want my stomach to get bigger. I have looked at drnatura.com, because i thought it may be the solution. It seems to be a reputable detoxifying system. It does not involve dieting in anyway. It is strictly colon cleansing and another product i cant remember. It seems safe and effective. I dont want to spend money on this though if it is not the problem. please help me out. If you have an questions let me know. I am ready to answer anything! THANKS!

also, I have a six pack when i suck in my stomach. (which i suck in always when i am at school, home, anywhere. So much that i do it without noticing.) I didnt know if this was normally. Because of this i didnt know if it was fat, or what. I cant figure it out. thanks to everyone who has taken the time to read this!