I agree everyone is different. I lost 15lbs In the first 5 months of this pregnancy. I'm 25 weeks now and have barely gained 5lbs. Baby is healthy (actually weighs more than normal) so my doctor isn't concerned.
Everyone is different. I don't gain. I always lose weight. It's about your baby gaining the right amount of weight. If your doc isn't concerned, you shouldn't be either.
Its recommended to gain 4.5lb in the first trimester and about 4.5lbevery month after that (27lb all up)... I've gained about the same as you at 20-21 weeks but in saying that, I also gained 20kg (44lbs) with my first and lost most of it within six months after birth. Remember, its not all weight that YOU'RE gaining... There is the weight of bub, the placenta, extra blood, fluid retention etc that will all disappear once bub is born.
Just eat healthy as best you can, drink lots of water and it'll all be fine ☺