Small bellies are usually okay
Every woman is different and every pregnancy is different. I had a very small belly while pregnant with Avery. I didn't wear any maternity clothes until I was 6-7months pregnant. Even then it was just my pants. I didn't need maternity shirts until I was 8 months along!
If it's your first baby your belly can be smaller because your stomach hasn't ever been stretched out before. Also, having a very strong core can kind of hold the baby in tighter. So if you're very fit then you could have a small bump. On the other end, it can take longer for curvy women to show because the baby has to be bigger in order to start popping out.
If your doctor says that your weight and growth are on track, don't worry about a small bump.