C.H.
I think your baby sounds just fine. The docs like to make you think there is a "normal" (read: growth curve), but every baby is so different. My baby gained well until 4 months and then plateaued until 10! She was healthy, eating, pooping, active, etc... just a peanut. Today she is still way below the charts, but gaining fine. We did have lots of tests done for thyroid and other issues, but nothing was wrong. Get her tested if you need to, but know that she could be just doing things at her own pace like many other babies.