Your OBGYN should be measuring your belly at every appointment. Talk to him/her about this. I think it depends on how you are carrying and your pre-pregnancy weight/figure. Some times my belly looks like I'm just chubby and at other times it is really hard and looks like the typical pregnant belly. As long as baby is okay, it doesn't really matter how we look- normal or not ;)
Were you bigger before pregnancy? How much weight have you gained. It could just be fatty tissue that you're able to suck in and not actually baby or your uterus. Some women gain weight in specific places. My weight gain is always is my thighs.. I'm 21+4, first time mommy to be and I've gained 10 pounds... I was 158 pre-pregnancy, 168 now. Buuuut only my family really can tell I'm pregnant. I can suck in some of my tummy too... Not anything below the belly button... Just around my rib cage. But I think most of my fat deposits (from the extra 10 lbs) are in my thighs... At least that's what my husband said. Lol