Small Uterus?

Last week I had an ultrasound and my baby was a perfectly normal size forhow far I am. But yesterday I went to the doctors and he said my uterus is not growing at a normal pace. Like I am 16 weeks and my uterus is the size of 12 weeks. He said not to worry because the ultrasound showed that the baby is growing normally. And he said next month when I come at 20 weeks my uterus has to be at least 18 weeks. But I can't make it grow! I asked why it's small he said I probably had a smaller uterus to begin with, I have a strong pelvis, or it's just taking longer to catch up in growing.

Did this happen to anyone else??? What happened to you?
What could happen if it doesn't catch up in growing? Why would this happen?
I'm scared... help!!!!