Happened to me. I only gained 25 pounds during my whole first pregnancy. And it was always like 3 pounds on 1 week or 0 pounds In one month.
I'm 22wks and have gained only 6lbs.
Yes. I haven't gained weight PERIOD....but my baby's weight is good and she the right length so I'm okay with that!
I didn't start gaining weight until 33 weeks. It'll hit you at the end. I'm almost 36 weeks and have gained about 16 pounds.
If baby is growing fine, then that's all that matters. You don't need to gain a lot to have a healthy baby.