Hey! I'm 21 too!! I lost 4 pounds at the begginning of my pregnancy and didn't start gaining until the second trimester. I started showing at about 15 weeks and (like you) thought it looked more like a gut than a baby belly!! It's hard to adjust at first. If your doctor is not worried, then please don't you worry!! Stress is bad for baby. I promise you will grow. I'm due in 5 1/2 weeks and have barely gained 18 pounds. My midwife says its great (I was a healthy weight to start with so I don't need to gain extra). If your pre-pregnancy weight is below what it should be for your height, then you should gain a little extra. Otherwise, don't worry. The baby will not starve unless you do. So, if you're eating, so is he/she. I know how fun it is to start showing. Soon enough you will.
Congrats!! You're in for such a wonderful experience. I've loved my pregnancy. I'd love to talk! Send me a private message sometime!