It's fine. I remember when I was about 30 weeks, and I thought I was HUGE, most people had no idea that I was pregnant yet, lol. I was in maternity clothes!!! And it took until I was over 35 weeks for a coworker of mine to figure out I was pregnant. So relax, it'll happen :)
I will be 21 weeks Sunday, and though maternity pants are more comfy (while sitting down) I can still fit in everything I wore before I got pregnant. It's totally normal not to show yet, but if my experience with my son was anything like this time around, I won't plan on getting very big.