Hello,
I started with a higher BMI as well (not hugely overweight) and I also only gained 3lbs by 18 weeks. I believe it's because we already had a bit of fat to start off with and if you were like me you started eating better that I didn't gain fast.
Let me tell you though that I have almost gained another 3lbs from 18 weeks to 22 weeks (22 weeks for me is this Monday). It seems like in the last couple weeks I have been hungrier than the previous 18 weeks. I am still eating healthy with my binge days just like I have done through the whole pregnancy. I remember reading some where that most of your weight gain doesn't happen until after 20 weeks which for me seems true.
Everyone's bodies kinda do there own things during pregnancy. As for the doctor not saying anything about your weight gain my findings is they usually don't start worrying until your last trimester unless you are gaining too much. As long as the baby is growing at a steady rate they won't say anything.