I'm sure your fine, your size has next to nothing to with your baby's size or health.
I wish I had a pic from my first pregnancy to show you but I didn't start showing until 28 weeks, I still had a 4 pack up until 26 weeks so when I say not showing I really means not showing AT ALL. This is a pic of me at 19 weeks with my second baby (you usually show a little sooner when it's not your first but I wasn't showing at all)
and this is 20 weeks with my third, a little bigger.
All three of my babies were healthy and big, my first was 8 lbs 3 oz born at 39 weeks, my second was 7 lbs 14 oz born at 38 weeks, and my third was 9 lbs 8 oz born at 38 weeks. Belly size is virtually no indication of anything to do with the baby, it has a lot more to do with your build.