I didn't start gaining weight in both my pregnancies until 22 weeks
I'm 21 weeks as well and I've only gained 8-10 pounds. You're just fine.
I'm 20 weeks and have gained 15 pounds so far, which I feel like is a lot lol
I'm 39 weeks and I'm still not up to my pre pregnancy weight. I lost 30 lbs in my first month and still negative 5 lbs. As long as your baby is growing then there's nothing wrong with you not gaining weight, it's taking up alloy of calories to help it grow that's why your intake is supposed to increase.
Congrats! I am 27 weeks pregnant and in the beginning I started to lose weight. I lost 13 pounds before I started to gain. At the end of my 2nd trimester I gained 4 pounds and I went to the doctor the other day and gained 4 more pounds. But most of my weight comes from the baby. So I wouldn't stress to much about your weight as long as the lil one is growing then that all matters.